Flak weapons can help shoot down enemy planes before they can make an attack run on your unit once they are called in by a Forward Observer. When building your army there are several things to consider:
Do you have enough Flak? It takes 6 “hits” from a Flak weapon to remove a plane from play once it arrives on the table and starts its attack in a target. Bear in mind because planes are “fast” you incur a -2 to hit right off the bat meaning you hit only on a 5 or 6 on the dice roll. This is not factoring in you may have the “long range” penalty (over half range) that could bump it to a 6 on a to hit roll. So, you are hitting on average 1 in 3 shots and perhaps as bad as 1 in 6 on average. Sure the dice gods can bless you or curse you further depriving you of a single 5 or 6 on your dice at that critical moment. Even a single Quad Autocannon with Flak in your army would hit at best 2 or 3 times on average. This is only about 1/3 to 1/2 of the way of taking down an enemy plane!
So, you have to consider adding in other weapons to your list to ensure that your Flak does its job against planes sticking with the laws of average above you would need 18 dice on average. Very daunting now to consider as a single quad AA can put 8 dice on the table, but you are limited in selections so you can’t stack up such weapons unless you use more than one platoon. So, you will either need to take up other slots with Flak such as MMG pintel mounted weapons (each MMG dishes out 4 shots so that helps stack up the dice quickly but the range is a bit more limited than an autocannon).
Also, you will have to factor in that you will take losses in the game, so what could be 18 shots could work up to 21 or more shots to ensure you have enough backup to cover any losses you have.
Now I will pause here to also remind readers that every hit with Flak reduces a hit by the aircraft so this works out between 1-5 (because at 6+ you have driven off/shot down the aircraft). Still, on average most strikes will be 3d6 hits with a smaller chance of 2d6 hits. The 2d6 is more appealing as you could cancel or reduce the hits down, while the 3d6 will be a bit more troublesome with higher rolls. Also note, you can half the total number of hits by taking a "down" action but then removes the unit as a threat for a turn as they are dealing with the down action order. This would be a last result and worst case scenario.
And as a side note, if you have Flak you should avoid using air support in your list if possible otherwise you could risk shooting down your own plane. It is a 50/50 chance that a standard unit could shoot at an aircraft on the same side; these odds get better with inexperienced and slightly worse with veteran units. Still, you can’t rely on that and hope the dice gods are good to you, that is just supplemental shooting more than anything to protect you from their own air strikes.
So, when playing keep an eye on your opponent’s list and learn what he has for Flak in the event he has air support. I assure you once this happens once or twice your opponent will avoid this early mistake in list building.