These days few people have personal secretaries, but back when they were common they really were personal - once you got a personal secretary she (nearly always she, I feel like we should acknowledge sexism even though it is irrelevant my point) would follow you (nearly always male), as you moved job to job and up the ladder. She went with the you because once you spent a few years training her to how you worked, a new secretary would greatly limit your effectiveness.

These days a large part of what people relied on secretaries for a computer can do faster, so only at the highest levels do you see them. There are still secretaries at the low levels, but not nearly as many, and they are not doing the same tasks.