Some use them to keep a close watch on the demand for their line of work or gather information on compensation to arm themselves when negotiating for a raise.
为什么or并列的是不定式to keep和(to)gather,而不能是use与gather并列呢?
这是根据句意来判断的。即:
Some use them to keep a close watch on the demand for their line of work or gather information on compensation to arm themselves when negotiating for a raise.
=Some use them to keep a close watch on the demand for their line of work or ( use them to )gather information on compensation to arm themselves when negotiating for a raise.