There are several flag types. I think this is everything:

+15 Health per second

+15 Mana per second

+15% Max Health

+15% Max Mana

+20% XP Gains

-15% Cooldowns

+10% Damage taken by enemy team

Gold Mines

Artifact Shops

You can mouseover a flag to see it's bonus (it'll be in the tooltip at the bottom right), as well as look nearby for gold mines, artifact shops, or portals (a flag can have a bonus and one or more structures tied to it, as well).

Sometimes depending on the map and game type, one of the flags in the middle area may have gold mines on them, but across most maps, the middle flags have XP buff, Max HP Buff and Max MP Buff, these can mean the difference in dying/living. All the flags have tooltips which state what they do. The other thing flags do is allow you to teleport to them using the scrolls so you can get around the map easier or ambush enemies.

Its very important because as others stated, the higher war score, the higher level creeps you can buy. Usually its a race to get giants, and thats a big boost to your offense.

1st team to war score 3 and buying currency will probably win the game, unless they have already fed the enemy team loads.

FYI getting flags is more important than killing a wave of creeps.  The flag will give you more experience and will help your team with the benefits of the flag and the warscore.  Stopping to kill a wave of creeps before getting the flag is bad, unless you have a quick cast Area of Effect (AOE) damange spell / ability that will take care of them quickly (ie. Oaks surge, Unlcean Beast's ooze, hammer slam etc.)

j

this i have to disagree on... use the towers to kill the creeps, don't leave even one wave untaken. at least one of the team should get the wave, get flags inbetween sometimes you'll miss a wave, but your teammates should catch them... again use towers if your slow at killing them.

by losing a few waves especially in the beginning will set you back one level to a team that just farmed... that one level will give them level 5 one level before you, this means they will be stronger... if they gain 2+ lvl's more than you (depending on build) they can dominate whole map.

you might lose 1k WR in beginning if you are weak in that phase...but you should be able to get that back when you get stronger than the enemy...

this being said, WR3= cur1 WR4=archer towers WR6=forts WR7=trebutchets (if your playing crucible)...meaning WR is damn important. its just that by having a lower level because you missed waves should lead to you not being able to keep those flags and the enemy catching up and running past you in WR.

also reinforcements aren't a must, it all depends on the enemy lineup, if they are good vs reinforcements it might be suicide to actually buy catas/giants before you can keep the enemy off your reinforcements. (again, if enemy counters with levelling up and gain a quick 5lvl lead you might simply be steamrolled even tho you have some giants).

I have seen been in too many games where a person (typically a regulus) will just sit there and auto-attack to farm grunts in a lane and stand an inch away from activation of an unopposed flag.  The ironic thing is that they are ranged attackers so they could easily cap and farm at the same time.  At the beginning of the game most creeps don't make it to the tower unless an opposing demi is taking our your creeps which means the flag that is there is opposed versus unopposed.  A capped flag benefits the entire team with warscore and/or +abilities where farming creeps just benefits the individual. This is especially true for the + gold, +xp and +hp/dmg flags.  Take for example a 5v5 map, grabbing a +gold flag 30 seconds faster will generate an additional 300 gold for the team, +xp for the individual.  This also prevents the other team from getting that 300 gold so the total difference is 600 gold (plus war score).  This, in my opionion, outweighs a creep wave (or 2).

I agree that you shouldn't miss multiple waves of creeps however missing 1 to cap a flag will generate more XP for the individual and that individual will be able to grab the next wave.  It is a lot easier to cap an unooposed flag instead of farming creeps and an enemy demi walks up preventing the ability to cap the flag.  On another note by the time a non-mine regulus takes out a wave of creeps another is almost there. 

It's rare in a game where i've seen a 3 level difference much less 5 but that is a good point.

And to support your argument, the neutral portal flags only matter for war score until cats/giants are bought.  I thought of mentioning this before, as long as you are even or gaining in war score, it can be beneficial to let the other team keep a neutral portal flag just for the creep farming. 

Anyways, good points Ersk.

thanks!

j