I do the same when I'm growing outdoors (unless it's on a balcony of course). I like to get the plants settled into their final large pots indoors with good compost/soil and I'll (sometimes very awkwardly) cut a larger hole/s in the bottom of the pots before placing them on a patch of prepared ground outside.
The main reason I started doing this was just to give the plants access to moisture in the ground for the rare long hot summer days where the pots would otherwise dry out, which has tragically happened to me a few times when growing on a balcony - I
usually remember to give my plants a reserve of water
before leaving for work on a sunny day now
.
I feel that this gives the plants a significant head start due to the extra heat and less damage to the roots vs. planting directly in the ground. Also, even in poor soil, I believe the native mycorrhizal fungi in the soil help provide some extra disease resistance and nutrients.