Pinning the firing pin down is done to prevent the firing pin from rising upwards and barely hitting the rim of the cartridge. This problem usually shows up in well used older guns that have developed sloppy tolerances.
The bolt handle is supposed to hold the firing pin down in it's slot, but sometimes does not, causing misfires. You can look at the rim of a blooper, or total misfire, and if the firing pin hit mark is way at the very top edge, this is an indication of the firing pin rising up in it's channel.
Most 10/22's may never experience this mis-firing problem. Pinning the firing pin down will eliminate the high hit problem. It's very simple to do, especially for a machinist. This link has some photos of how I do the job.
10/22 Bolt Pinning - RimfireCentral.com Forums