There are various types of hill training that can be incorporated into every phase of training. For example, longer hill repeats with an honest pace up AND down the hill during the base phase is an excellent way to build strength. On the flip side, hill repeats during the pre-competitive phase can resemble that of an explosive nature such that you are sprinting up the hill and merely jogging down for more recovery.
Lastly, a great way to build strength and have some fun is to run a Tempo Run up a large hill (or mountain). The effort makes the view that much better than if you drove to the top.
Who: The more seasoned runner
When: Base building phase and pre-competitive phase
What: Short to medium length at a relatively high intensity
Where: Hills with a gradual incline of roughly 4-6% gradient
Why: Increase strength, speed, and lactate threshold
10x 300m hill with jog back down the hill for recovery