This mid-season break is called Hiatus and it's general purpose is to make the show run longer. However, some TV channels reserve episodes for airing during ratings sweeps while calculating their television advertising fees based on their programs' ratings during that period.
From Screener TV (emphasis mine)
As with most things in the television industry, what it really comes down to is money. With the exception of premium cable networks, most networks make money off of their television shows by selling advertising. The advertising companies use ratings to decide how much a 30-second commercial is worth during a certain show, based on how many eyes will actually see that commercial.
Generally, it’s in the networks’ best interests to make sure their best, most attention-grabbing episodes air during sweeps, because then more Nielsen families will watch, then the networks can sell commercials for more money. That’s why you’ll most often see big-name guest stars or shocking twists during these periods. Shows often go on hiatus right before these breaks so that the networks can save their best episodes for sweeps weeks. Shows return from hiatus with a big promotional push and lots of press right as the advertising companies start paying attention.
The Guardian seems to have the same point.
But put simply: channels only want to screen their precious shows when the maximum number of people are watching – which allows them to charge advertisers more for the advertisements they put in the middle. There are other factors, but series tend to start in September/October, and then run until the end of November (Thanksgiving). They may return for a Christmas special, but otherwise disappear for six weeks – this year more, thanks to the Winter Olympics – replaced by repeats, until people can be trusted to be back on the sofa.
Each year, Nielsen processes approximately two million paper diaries from households across the country, for the months of November, February, May and July—also known as the "sweeps" rating periods. The term "sweeps" dates from 1954, when Nielsen collected diaries from households in the Eastern United States first; from there they would "sweep" west