For this example, I will use an arm day with just biceps/triceps.

Your normal routine probably looks like this...
2x warmup and 3x working sets of straight bar bicep curls
3x hammer curls
2x warmup and 3x rope pulldowns
3x skullcrushers

That's about what my routine was looking like for years...not necessarily those exercises but only 2 different exercises for biceps and triceps and 3 working sets of each.

This is an example of what I have been doing lately

2x warmup sets of and 2x working sets of straight bar bicep curls
2x working sets of hammer curls
1x working set of preacher curls
1x working set of concentration curls
1x working set of (insert random bicep exercise)
2x warmup sets and 2x workin sets of rope pulldowns
2x skullcrushers
2x close grip bench press
1x (insert random tricep exercise)

The philosophy is too not do too many sets of the same exercise. It could be argued, but of course not scientifically proven, that by the 2nd set of one exercise, your muscles are already accustomed to what you are doing to them, and will not be shocked like they were on the first set. This way of lifting is to constantly keep the body guessing what is coming next, and it is also exciting doing many different exercises, and not walking up to the rack thinking "okay i gotta do 3 of these."

There are some exceptions to this philosophy I believe, though. There is no substitute for deadlifts and squats and i will always do 3 or 4 of each of those at a time, but you can do one drop set, one straight set, etc. Or maybe one set of squats, take a lot of time when going down on squats and burst on ur way up.

In conclusion, don't get stuck in a rut. Going back to the arms example, straight bar curls and hammer curls are the best exercises for biceps, but too many of each will not keep your body guessing, and you will likely hit the plateau after a while. I am not a scientist. I cannot prove any of this. i can only look in the mirror and see the improvements I've made recently after not making any for a long time.