In relativity, we do not talk about just space anymore, but space-time, with time being just another dimension. An observer A at rest in an IRF who considers himself at rest at t=0 x=0, has a far away clock at t=100 for example moving towards him, i would like to say at 1second per second, but i guess that wouldn't make sense. It is however moving towards him at SOME pace. Someone moving at vrel relative to observer A, sees him moving towards the clock along the x and t axis. There has to be a reason for that, because otherwise it seems logical to assume that we would just stand at a fixed(static) coordinate without moving towards either the x or t axis.