That's a broad question, as it could cover just grabbing an enemy rifle because yours failed, or sticking a captured handgun in your belt, or the systematic reissue of captured equipment as the Germans did in both World Wars.

In general, it was rooted out of immediate need (battlefield pickup) or supply shortages (Germany overhauling and reissuing captured arms). In the case of German reuse, it helped alleviate shortages of German arms, or improved supply issues. Reusing Russian arms on the Eastern Front was common.

Finland made an art out of reusing captured equipment, as they had long standardized on the Mosin Nagant rifle for their primary battle rifle. Not only would they reuse captured Soviet arms, they would often rebuild them with new stocks, triggers and barrels to meet the high standards of accuracy they demanded from a rifle; although many WWI and WWII rifles are simply found with Finnish property stamps, as opposed to being totally rebuilt.

I don't have figures but at Hamel in 1918, the Australian soldiers involved were trained and directed in the use of German rifles, grenades and machine guns. During their advance on the town, captured machine guns were turned on the retreating Germans. John Laffin points to this addition of firepower in the field as an important element that contributed the the Australian victory.

The Battle of Hamel: The Australian's Finest Victory by John Laffin

It was rather common in several different ways.

• Souvenirs - Officer pistols were often seen as 'prestige' items and as pistols were rarely issued to grunts it was a way to acquire one. Pistols were not that often actually USED in a battle but they were cool souvenirs and they made the carrying soldier feel a little better protected. You can see this illustrated well in Band of Brothers where many of the soldiers are really eager to get their hands on a Luger.

• Necessity - It goes without saying really that if your own weapon runs out of ammo or breaks or jams it is a smart idea to just grab whatever you can. This is usually a short-term solution as you will be unlikely to be issued spare parts or ammo for it if you were to hold onto it.

• Sanctioned by the army - You can see this a lot with the Germans. They took over most of Europe in WW2 and captured vast stocks of weapons from all the armies they destroyed. The Germans employed not just small arms but artillery, tanks, etc.

For example you can check out the Marder III. http://en.wikipedia.org/wiki/Marder_III This was a German tank destroyer but it was built upon a modified Czech chassis to use a captured Soviet gun - so its actually a hybrid of 2 captured weapons!

German infantry also were fond of using captured Soviet PPsH's as they were considered very reliable.http://en.wikipedia.org/wiki/PPSh-41 The Germans looted so many of these from prisoners and corpses that the army even modified them in large numbers to fit standard German ammunition. It was not unusual for a German soldier to be given a foreign weapon as his standard issue.

• Partisans - Partisan and resistance groups were happy to get their hands on ANY weapon really so captured ones were usually better than the hodgepodge of civilian weapons and home-made stuff they were typically armed with.

• Subterfuge - If you are sending in troops disguised as enemies or behind enemy lines it is good to equip them with foreign weapons to complete the disguise and also because they are more likely to come upon the correct ammunition for this. A good example can be seen in the Battle of the Bulge where many German commandos went behind US lines dressed as American soldiers complete with equipment - some even had US tanks!