what is the point of the maths puzzle? Instead of a maths puzzle, why can't we have miners collect transactions, then just make them wait 10 minutes before accepting the first block broadcasted?
Proof of Work (the proof that "maths puzzle" was solved) serves a variety of purposes.  The primary purpose being to determine the "first" miner whose block should rightfully be accepted as the next block in the chain.  If there was no cost to proposing the next block, there would be far too many candidate blocks and the network would split too often.

With Proof of Work, the network can determine which miner has earned the right to mine the next block by having expended the electricity to produce the hash that meets the difficulty level.