Lots of Spanish phrasal verbs need to be followed by "de." (Others by "en". and others by "a".)
English also has phrasal verbs: Examples include: "go out; look forward to; watch out; look at; switch on; look after; sign up; back off; and many more.
Just as in Spanish, the meaning of the phrasal verb is usually different from the "literal" meaning of its component parts.
For example, what do "look up" (a word), or "sign up for" literally mean. Literally they make little sense.
This Spanish phrasal verb simply means "go shopping."
ir a comprar means going to buy - use it if you intend to buy a specific item.
ir de compras means go shopping - use it to mean you are going out to buy stuff, possibly at various stores.
- Voy a comprar estos zapatos. I am going to buy these shoes.
- Quiero ir de compras con mis amigos. I want to go shopping with my friends.