This money is actually known as Bae Allowance – money a guy gives his girlfriend at the end of each month for her random needs.
There is nothing stopping anyone from giving money to a girlfriend. Heck, there's even no rule about how frequent it should be or how much it should be.
It is not illogical for a man to want to spoil his woman in all manners he deems fit. If a monthly allowance is one of the ways he chooses to appreciate his woman's presence in her life, then, no one should give such guy grief for that.
For the guys who feel comfortable doing so, it is admirable and nice. And you should never feel discouraged by people to stop if you see no problem with it.
Bae allowance is not a relationship standard
Bae allowance may not be wrong but it should never be a relationship benchmark for anyone. If you date someone who gave you a monthly, weekly or daily allowance, it is important to understand that such person did it as a largesse and not a compulsory or widely-expected relationship standard.
Bae allowance is not on the same pedestal of relationship expectations as communication, honesty, gift-giving and others.
This is why every money [whether monthly or less frequently] between partners need to be appreciated for the gifts they are, and not demanded as if they contractual obligations.
As we already said in another post here, your boyfriend does not owe you the duty of gifting you money, just as you do not owe him the duty of cooking his meals or tidying up his apartment in your visits.
People should do these things because they are capable and want to do so. No one should be compelled to do them.
There are relationship things you can demand because you are entitled to them; a monthly allowance, as nice as it is, isn't one of them.