Keeping up on the content you enjoy through YouTube is easy to do by subscribing to the channels you watch the most, but if you use YouTube quite a bit you may have quite a few subscriptions which populate your channels. Collections was a way for users to group these subscriptions into categories which could keep similar channels under one tag or name, making it easier to find the lists of subscriptions you like to view at any given time. While the feature likely gets use from some people it appears it is either not getting enough use or Google may have something better in the works as they will be shutting down Collections near the end of May.
According to the YouTube support page for Collections, Google will discontinue the feature on May 20th, 2o15 which gives people who are still actively using it just ten days left to get their use out of it before it disappears. Why it's going away is unclear at the moment, but Google does mention that while they will be discontinuing Collections on May 20th, they'll also be working towards making subscriptions more enjoyable for users. This could mean that Google is focusing efforts on a new feature to replace Collections, or rather they may be just revising a new way to group subscriptions together that makes the need for Collections obsolete.
For now, Collections is still up and running so it can still be accessed, unfortunately there is no explanation from Google on whether or not people's subscription Collections will be grouped in some other way once the feature is shut down. One could only assume that the groupings will cease to exist entirely until Google comes up with some other method to lump them together in a similar manner. Without Collections, subscriptions will simply be listed as they are for anyone who never used Collections in the first place, which is either channels that are the most relevant at the time, channels that have the most activity, or channels that are listed from A to Z. If you're looking for a way to keep things organized, alphabetical order may be the easiest way to find the channels that you watch regularly without another way to group them together.