Expect a heavy dose of close combat in 2fort. With plenty of firepower coming from both teams in such a small map, it will be a smart strategy to have Medics to keep your teammates from dying less often. Players can take a lot of hits outside the forts where the battle can get ugly or in the underground sewers where Soldiers, Heavies, and Pyros can easily deal damage thanks to the narrow corridors and tight space.
Penetrating through the opposing defense is another reason why the Medic is important in 2fort; with his ubercharge ability, he can charge himself and another player (most times a Heavy) to be invincible for 8 seconds. The Heavy/Medic combo can be a good way to break down a crowd of enemies defending their base. This strategy can also be seen as a decoy, as enemies will direct their attention to the ubercharge; then they will be more vulnerable to open fire from your teammates behind you.
If the other team is using the Heavy/Medic strategy, try to take out the Medic with a one shot kill since they may ubercharge right away if they don't die from a hit.
The Heavy and Medic make a dangerous pair, but since the other classes here also are very important to capture the flag, like the Scouts and Spies, it's wise for the Medic to team up with them too. They're two classes with low health, and anytime a Medic is with them, they obviously have a less chance of dying and easier time moving ahead.
Another teamwork strategy is to pair up with another Medic who is charging a Heavy. You give this Medic a better chance to survive and now him and his partner are even more of a threat; plus, you're their backup plan in case someone dies or their ubercharge bombed on them.
Two Medics working together may sound weak, but in truth, they can be a dominant force against a defense. With this strategy, you have two strengths- both of you can heal each other and create your own ubercharge. Imagine how that can cause alot of problems for your opponents. You will not die easily and this should give you ample opportunity to confidently attack the enemy base; your primary and melee weapons become quite effective under this occassion. The two Medics strategy is capable of destroying sentries and enemy groups.
So remember that the Medic is not just a healer, but a powerful weapon as well, and can help open the pathway for the rest of your teammates to move up ahead.