There are a couple of possible reasons.
I don't know what red leaf extract is?? Is it red tea extract?
I take it that the stationary phase of the TLC is silica gel? It is quite likely that the extact contains a number of nitrogen containing compounds, such as alkaloids (it will if it is tea). These compounds are quite polar and tend to stick well to silica, resulting in blobs that smear up the TLC plate with little separation or do not move at all.
By adding ammonia to the mobile phase this makes the silica less effective at holding on to the alkaloids as in effect the acid silica gel has been neutralised. This tends to give better separation as you get spots that move rather than getting a smear up the plate.
Often the amount of ammonia needed is a matter of trial and error. Some chemists also use other amines such as propylamine, but again the choice and how much is a matter of trial and error.
You can also get TLC plates that are coated with alumina, which is already basic.
(I used to have to make my own TLC plates and got quite adept at adding ammonia to the water slurry that the plates were made from - but it meant I had a lot of smelly TLC plates drying!)