On several occasions I've departed the airfield only to find that I had major instrument errors. These occurred in an airplane which used large static ports (massive, by today's standards), which had accumulated moisture in strong winds and rain. There were no static drains.
Static drains do two things; they drain the static system of contaminates, and in many cases, are used as the alternate static source.
In some systems operating under IFR, opening a static drain means the pitot-static system needs to be recertified, retested, or reinspected. Be sure you know what applies to your system.