Όταν βράσουν τα μακαρόνια, τα στραγγίζω στο σουρωτήρι και τα βάζω πάλι στην κατσαρόλα.

Because Greek is focusing on the pasta reaching the point of being boiled/cooked, and then the next action happens.

• βράσουν is the aorist subjunctive (also called the perfective non-past) of βράζω
• after όταν, Greek often uses this form when it means when/once something has happened
• in a recipe or sequence of actions, this is very natural: first the pasta cooks, then you drain it

So όταν βράσουν τα μακαρόνια is basically when the pasta is done / once the pasta has boiled.

If you used βράζουν, that would sound more like when they are boiling / whenever they boil, which is not the best fit for this step-by-step instruction.

It is the 3rd person plural aorist subjunctive of βράζω.

You may also see it described as the perfective non-past. Those two labels are referring to the same basic form in Modern Greek grammar.

Breakdown:

• βράζω = I boil / I cook by boiling
• βράσουν = they boil / they cook, in a subjunctive-like form used after words such as όταν

Even though it looks a bit like a future idea in English, it is not a future tense by itself. The future-like meaning comes from the context: when the pasta has boiled, then...