STUN (Session Traversal Utilities for NAT) is a mean for a device which sits behind a NAT firewall to learn the public IP address of the Internet connections which is beding natted. I.e. your laptop may have been assigned a "private" IP address as per RFC 1918, for example 192.168.1.123. Any software on your laptop asking the operating system "What is my IP address" will get 192.168.1.123 as an answer, which isn't helpful for applications which try to use protocols that were simply made without NAT in mind.
The NAT unfriendliness of protocols usually kicks in whenever a client connects to a server and is supposed to tell the server its own IP address in order for the server to be able to initiate connections to the client (as opposed to just sending answer packets on the same IP connection; this is handled by NAT routers.
STUN is a means for a client to learn the public IP address of the NAT router through which the connection is natted in order to use that information when registering with a server that wants to call back now or later.
The 95% use case indeed is VoIP telephony, especially SIP.
If you found that the STUN requests originate from the Safari process, it cannot be either Skype or any other conferencing software, as that would typically spawn a separate process.
STUN (and TURN) might be used by Safari for WebRTC connections; though that should only happen while you are using any such sites, i.e. video chats in the browser.
Otherwise - to answer your original question - there is
no reason to allow those connections unless you intentially use any services which need them and
I would want to find out what piece of code (browser plugins?) try and make those connections
It may turn out it's something you just did not think about. But it may also turn out to be something harmful.