The relevant point isn't how important the outcome of Team A versus Team B is, but rather the amount of influence an individual has over that outcome. A normal

The relevant point isn't how important the outcome of Team A versus Team B is, but rather the amount of influence an individual has over that outcome. A normal individual has very little effect on the outcome of sporting competitions, but an individual sports fan does get satisfaction from choosing a team to cheer for. Likewise, a normal individual has very little effect on the outcome of major government policies (e.g. federal policies in the USA), but an individual voter does get satisfaction from associating with a particular political group, following their events, researching their arguments and rebuttals to opponents' arguments, etc. It's true that major government policies actually are extremely important, but that doesn't mean that individual political participation has much of an influence on outcomes. It is very common for people to have a strong association with some political group and act similarly to sports fans (e.g. "my group is great, your group are a bunch of hateful cheaters"). It's not as common for people to spend effort and resources attempting to increase their actual influence on political outcomes (things like having constructive discussions, organizing events, campaigning, or even running for office), because people correctly realize that most of the time the effort and resources spent would be larger than the resulting effect on the outcome.