This phrase describes a situation that sometimes happens. It uses the future tense "will", but it doesn't describe something that happens in the future. Instead, it's something that has happened in the past several times and might happen again. In the example above, it means that the speaker drinks beer on very rare occasions - for example, once a year.

Here's another example of "will" used to describe something that sometimes happens:

But sometimes you'll be flipping through the channels and come across something really interesting.