I assume you mean a front fork lockout? Although the answer is bound to be the same for either front or rear. The thought is that you use it locked on smooth surfaces, that is streets and trails etc. And UNlock it when going a bit more off-road or over rougher surfaces. I personally keep my front fork suspension unlocked at all times. I run into rough pavement and from time to time have to cross a dirt, stone or grassy section. I am also old and cannot soak up the bumps very well. I'll take the performance hit as a trade off for the comfort.