Mainly the valves. The tuning slides help with a number of things, including the fact that the harmonics (notes above the first ocatave) are not precisely in tune with the fundamentals. A trumpet typically has a trigger lever or a loop for your finger on one or two of the tuning slides.

You use it as needed. If you're playing a really fast passage, you'll likely skip it, but shorter notes are harder to place the precise pitch anyway.

If you really want to see tuning slides in action, find a video of a good tuba soloist.