쓰다 covers using, spending and writing, and -(으)면 sets the condition: 필요하시면 쓰시면 돼요 (use it if you need it), 국내 여행 가면 쓰지 (we'd spend it on a domestic trip). Two set shapes appear — 쓰면 쓸수록 'the more you use it, the more ~', and 쓰면 쓰는 대로 'however much you use, that's how it goes'. A separate old-fashioned 쓰다 means 'won't do', which is the 지나치면 쓰나 line: can you really just walk past?