> this way i don't have to use root when building

`makepkg` will actively refuse to run if you are invoking it as root (unless you specifically invoke it with something like `env EUID=123 makepkg ...`).

> and can have program installed locally just for a single user which is how it should be anyway for most desktop use cases.

I do wish pacman would support a user level installations. It will refuse to install packages as non-root (which you can go around by using user namespaces and mapping yourself to root).