1) For End Users and Developers, Golang can mean fewer bugs
Golang is a "type-safe" language, meaning that all the programming code is written to be explicit and consistent, versus "untyped" languages like PHP or JavaScript, where programming is more implicit with many different ways to say the same thing.
It's easier for software tools to automatically analyze type-safe languages and find bugs. And those languages are also easier to read and review--since there's only one way to express something--and that helps too.
2) For System Administrators, Golang means easier install and management
Golang is a "complied language", meaning that after the programming is done, a single file is produced. So a System Administrator just installs one file and Mattermost works. To upgrade, they just change that one file. Easy!
Languages like PHP and JavaScript are "interpreted languages", meaning that a System Administrator would need to download lots and lots of files, install one or more programs to interpret those files, and hope that nothing breaks. When it's time to upgrade, she's got to make sure all the files and interpreters on her server are exactly the right version. It can be a nightmare to maintain.
3) So what's the trade-off?
The trade-off with Golang is that it takes more work to learn than PHP and JavaScript. The programming language isn't as forgiving to new programmers. You need to say things exactly the right way or the program won't even start, whereas PHP and JavaScript if you say things "kind of right" and things will "kind of work".
Does this help?