of the 4 torrents you downloaded 2 have leechers in addition to a seeder:leecher ratio better than 7:1 ... which is quite rare for bakabt.
so no, you don't have to download and seed random torrents just to maintain a ratio - you have to seed the stuff you downloaded to maintain your ratio.
even for torrents that show 0 leechers stating that it is impossible to gain a ratio boost from them is a lie. two of my highest ratio torrents have a nearly constant 13:0 seeder:leecher-ratio - while it is true that i'm seeding them for roughly 1.5 years now (because they have such a low seeder number) i also gained a ~50+ ratio on them, which translates to more than 2 full reseeds per month.