a betta can be kept alive with bare minimum conditions and bi weekly water changes in a decent size bowl or vase but it's still not healthy for them. a 5-10 gallon tank is always the better option

poorer conditions mean your fish is less resistant to illness like freshwater white spot disease(yes that really is a thing)
you don't need to stress the fish scooping it out once or even twice a week to change the water, with a tank this can be done without even bothering your fish
heavily planted setups and places to hide make them feel safer
filter not only cleans your water but also allows beneficial bacteria to grow which are appreciated by live plants and help eliminate threats like ammonia and nitrites. also current keeps water from being stagnate

TLR the sign of a happy betta is one who is quite active, if your fish is lethargic, sick looking or lifeless he is suffering