So you've removed the trichomes from the flower, and then just add it back in again afterwards? Certainly adding a whole bunch of them to a joint will get you higher, but in order to have that amount you would have removed it from other joints, it all evens out. It all evens out.
Removing kief from your bud then sprinkling it back on later only "evens out" if absolutely ZERO trichromes are lost or damaged in the process.
The problem is that this isn't true.
In breaking up the trichromes into "kief" some will get oxidized and you will have chemical loss there. When grinding some kief will remain stuck in your grinder where you can't easily scrape it out, and it will be permanently lost when (if) you clean the grinder. Some kief grains will float into the air and/or get spilled when opening your grinder up or scraping.
Now maybe these forms of loss will be small, but they won't be zero, so my suggestion is, unless you are deliberately trying to concentrate kief to create a form of hash, there is no good reason to remove the trichromes from your bud.
Back on the original question, grinders do three things for you:
-They ensure a UNIFORM FINE GRIND.
-They're generally physically easier to use than chopping up the bud with scissors or the like, particularly with hard/dense bud.
-They may catch kief that could otherwise be lost when chopping manually.
Of these three things, I think the first is the most important.
If you're going to use a vaporizer, many of them simply won't function well unless the herb is finely ground. Its harder to do that with scissors than a good grinder, and it can be next to impossible with just fingers. Even if you're smoking a joint or bowl, the herb will burn more evenly, and stay lit better if its evenly and properly ground. Appropriately ground herb is also physically easier to roll and pack.