Bonjour N. ! In this case, "on" refers to people in a vague way: we don't know who they are, how many, etc... However, we do know they are specific people who allow me to use the computer in that specific instance. "We allow" would be misleading, as it would include the speaker, and "one allows" would be way too vague and generic. "They allow me ..." is therefore the best translation here to reflect this "on". I hope that's helpful! À bientôt !