why do you need 75 posts to thank something because i want to read how to make spites but i cant because of the thing that says i need to thank it.

i geuss because they dont want spammers and people will be more ectice on forums trying to get it

I also do not get why is there a minimum number of posts before you can thank a post. Is something gained from thanking a post other than the fact that the content was really thanked?

Because if there was no requirement, people can just go to different account, or another account and thank every post you have on your main.
Therefore abusing the thanking system.
You used to get forum credits from them. But since someone was abusing it, it was removed. Hence the 75 posts still is being applied.
And it may not be removed as people can still abuse it to "look good" on their profiles.
Thanking someone's post does not give you benefit as they do not know who thanked who, so the thanking system doesn't do much.