What exactly is your 'garrison' used for and is it still relevant in the Legion expansion?

I just started playing WoW again, and haven't played since before 'garrisons' were in the game.

What exactly are the intended purposes of your garrison? And is the garrison still relevant in the newest expansion 'Legion'?

Your garrison is the main hub of your activities in the Warlords of Draenor expansion.

It provides you with the beginnings of many vital WoD questlines, crucial crafting (profession-based) resources, and is where you launch Missions (Followers and--later--your Shipyard fleet) from. All of these things contribute significantly to the core Warlords of Draenor experience. Further, a well-managed Garrison was an extremely lucrative source of gold, facilitating trading between players for highly desirable resources.

However, now that Warlords of Draenor is at an end and Legion has started, the importance of the Garrison has significantly diminished, similar to how the launch of WoD diminished and marginalized the importance of your Halfhill farm back in Pandaria. There are still a significant number of unique things (Battle Pets, Engineering toys, etc) that can only be obtained through the use of your garrison, but it is no longer a core piece of the present expansion's experience.