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Many depreciation techniques work on a time-basis.
Every year, items lose a chunk of their value.
Time isn't always a good way to measure depreciation though.
Sure, a fancy laptop you buy today will probably be an obsolete piece of junk in a few years, but there are many items that aren't like that.
Think about a ballpoint pen. You could buy one today, stick it in a closet for a year, and it will be just as good as a brand new one.
In other words, time isn't always a good tool for measuring how devalued an item has become.
Let's get back to that pen. How do you know when your ballpoint is no longer useful? It's probably when the pen runs out of ink.
As a result, we can think about the amount of ink that's been used in a pen as a proxy for how much depreciation the pen has absorbed.
When a pen's ink is halfway gone, we can say that the pen is only half as useful as it was before..
Many items are just like that pen. They don't wear out from time so much as from use.