Is it safe to delete this "Installer" Folder?

Yeah - delete it! Nothing special.

Insider: Are you able to cope with system errors? Because I can promise you one thing: if you don’t think you can manage without toolXY for a few days now and then, I cannot recommend the Insider Preview Canary programme to you!

But if you enjoy trying out new things, reporting bugs clearly and concisely via the 'Feedback Hub', and getting to grips with new tools, then you’ve come to the right place!

Insider program is great if you work in Tech/Ops to test things you have deployed in a live environment first on your machine so you can prepare in advanced or report any issues that caused disruption with one your systems. 
Another reason insider program is great is if you like to dive into bugs on OS, you can be the first to report while testing the new features.

If you don't do one of the two, then its not really worth going to it.