Proof of work gives you spam resistance basically. Add a computational cost to do something to make it computationally expensive to do. Eg ddos protection, your browser needs to solve a hashing problem to proceed.<p>It allows bitcoin to be decentralised (unordered message passing, eventually consistent nodes, no on off switch, no single point of failure) trust-less (no authority to complain to who can arbitrate or make changes), and resilient to attack .. all of this: in consensus on mining new blocks, in immmutability of the transaction history, in releasing new coins into the system at a predefined rate. To name some.
Without the difficulty increases you would get more blocks mined per hour as the mining hardware improves. At some point you'd have multiple chains at once that can grow quickly because its too easy to hash, so effectively forks.