1. Ammonia melts delicate plants (such as plants that come in tissue culture format) and is harmful to livestock. This is more important for aquasoil tanks that use ammonia rich aquasoils (such as ADA), where initial ammonia levels are very significant.
2. Having matured microbial community in the tank allows faster processing of organic waste pollutants. This maturation process reduces incidence of triggering algae. This take a long time without seed material (mulm/mix of old substrate from previous tanks etc). Avoid triggering algae in a new setup allows plants to settle in more smoothly. This is especially applicable for high light setups.
Cycling before planting reduces algae triggers, and creates a better, more stable environment for sensitive plants & livestock. Diatoms and green dust algae are common in new setups; so common that many experienced folks think that it is an inevitable stage that all tanks have to pass through. However, both these algae stages are greatly minimized or absent if tank is allowed to pre-cycle and mature beforehand. Especially for beginners that have a hard time handling algae, avoid the issue to start with is a smart move.
If you are using relative easy/hardy plants and non-ammonia rich substrates, then planting without pre-cycling is fine.
For tanks that use higher levels of light, its easy for plants in a new tank to be quickly smothered with algae if ammonia levels are elevated.
Ammonia is a great source of Nitrogen for plants; which generally use less energy to synthesise Nitrogen from ammonia than from say Nitrates. However, since ammonia is toxic, plants cannot store the compound directly in their tissue, and have to reduce the ammonia to Nitrate before it can be stored. This means that plants can only use Ammonia effectively for growth at the point of growth - having excess is not conducive and plants will not be able to uptake large amounts of it.
The downside of having ammonia in the water column is that it is toxic to both fish and plants. This is a key reason why delicate plants such as Utricularia graminifolia melt when planted freshly in new setups. The other hardy plant that melt in new setups often is Bucephalandra. Both these species are much better off in matured tanks.
Besides its toxicity, ammonia is also a great trigger for algae. This is especially an issue for tanks with higher light levels - where the tank can become quickly covered in diatoms and green dust/green spot algae making it difficult for plants to adapt to the new environment as algae start covering their leaves.
The trade off of instability due to ammonia spikes is not worth the marginal boost in plant growth that ammonia would give. In newly planted tanks, Nitrogen uptake is slow as plants that have not settled into the new environment would not be growing at maximum growth speed yet. Most of the ammonia present would end up being oxidized to nitrates over time.
Ammonia is best provided in the substrate zone - ammonia binds to soils. Having it in the substrate with the use of soils/aquasoil substrates allow plants the benefits of access to ammonia when they require it and it becomes a significant boost to growth. Ammonia in the water column is quickly oxidized to Nitrates in biologically matured tank.
Having ammonia in the substrate rather than the water column allows us to boost growth without having an additional algae trigger in the water column.
Bucephalandra are undemanding plants, but require a stable, clean, well filtered environment to do well long term. This easy plant often melts in new setups due ammonia and other organic contaminants.
The old method of cycling a planted tank was to add a light fish load of hardy fish, which then produce organic waste and ammonia; the tank was then left to build up the bacteria naturally over time. This can take many weeks, as more and more livestock was added slowly. Seeding the tank with seasoned filter media or mulm from the substrate of a previous setup are additional ways to kick start the cycling process. Depending on how it's done, this process can be tough on livestock. In acidic tanks (pH <7), ammonia toxicity is alleviated as most of the ammonia is in the NH4+ (ammonium) format due to low pH environment.
Nowadays, fish-less cycling with plants is the rage. This involves adding liquid ammonia regularly into a new tank; fully setup except for livestock or plants, to grow the bacteria colonies. Follow these 3 steps:
Typically, we would change 80% of tank water to reduce nitrates before adding livestock. Adding starter bacteria cultures (such as API quickstart) at step 1 greatly speeds up the process. Bacteria products work, but many are sensitive to temperature. Depending on how it is transported and stored, cultures could be DOA (dead on arrrival). That is the main risk/downside with bacteria products.
The type of aquasoil used matters a lot. Aquasoils with little to no ammonia release doesn't have the same impact as ammonia rich aquasoils. Milder soils like Fluval stratum or Tropica are not as harsh as say fresh ADA aquasoil.
What you also do not see is the daily water changes, replacing of melted plants and cleaning of algae behind the scenes. Hardy species that are planted straight into fresh soil generally recover even if they take the initial hit from ammonia. However, for many beginners that do not have good control of their CO2 and other growth parameters, it could result in losing entire batches of plants.