UPnP is helpful in certain situations where you need a device to poke holes in your firewall for particular services, the most common usage of UPnP in MikroTik is for online gamers who want to achieve an open NAT type for their XBOX One’s, PS4’s and other applicable gaming consoles. Some CCTV systems do offer usage of UPnP for remote viewing. UPnP is seen as a bit of a security flaw because it allows devices to poke their own holes, if a device becomes compromised it could open your network up to a lot of nasty stuff from the web, you should only really use it as a temporary measure whilst you employ a good port forwarding structure or where port forwarding is not possible.
Both written and video tutorials are now written for adding UPnP to your router using MikroTik RouterOS. UPnP is a nice easy one to do but should only be used if you absolutely can’t forward the relevant ports.
Written tutorial can be found here
Video can be found here