Yes, you can and we do. In fact, since diamonds (and in particular synthetic) are the hardest materials known, a lot of diamond is manufactured to serve in cutting tools. Also in some electronic devices iirc. Lemme google it actually, I don't quite remember details haha.
Okay, yeah. When you synthesize them, you can control their properties a bit better than when they form naturally. Such as hardness and thermal and electronic abilities. It's also not really important to have them be shiny since they're not going to be admired as gems but used as tools (but I think if they are very pure they should be transparent anyways?). Basically, you can synthetically create diamonds purer than naturally occurring ones and that's why they are so hard.