If you want to say something changes at a certain rate, you need to introduce a dependent variable under which it changes. If you look at landscape, and saying elevation changes, you don't need the coordinates as variables to associate height with and compare the different (x,y,height) data points -- only a set of {height1,height2,...} would be sufficient. But to do calculus by taking the small differenes (x-x',y-y',height-height'), i.e. looking at rate of changes of height w.r.t. something , you obviously need something (e.g. coordinates).

Ultimately as with most physical phenomena the most profound explanation is that "It's a simple way to describe how things work". That and anthropic necessities.

Relationships between events are almost certainly an anthropic necessity. First without change at all, of course there would be nothing of interest. Second an universe that's simply a random-looking collection of events {A,B,C,...} without some close functional relationship (e.g. {A,B=f(A),C=f(B),...}) also cannot allow a conscious subsystem (because conscious subsystems are defined as a particular kind of changing system, i.e. with close functional connections between the events that temporally evolve). There's an infinitude of mathematical constructions that may harbor consciousness, so some things are arbitrary.

Our arbitrariness is that there appears to be this continuous dependant real variable (time) which permits localizing events in a complicated way described by general relativity and quantum mechanics, and describing relationships between them (temporal evolution).