- There are two that go via Belarus -> Poland -> Germany and Belarus -> Ukraine -> Slovakia -> Czechia -> Germany
- Another two through Ukraine -> Slovakia -> Czechia -> Germany and Ukraine -> Romania -> Bulgaria -> Turkey
- Then are two in the Black Sea, South Steam and Blue Stream which go to TurkeyLet's say you believe (a) that Germany desperately has to buy gas from Russia in order to survive the winter (not an entirely unreasonable believe back then), and (b) that it's possible for Germany to use its influence to pressure Ukraine into some kind of Russia-friendly peace deal (as has in fact happened after Russia conquered the Krim in 2014). Then forcing Germany to have that gas delivered through the active warzone in Ukraine sounds like a good bet to make (b) happen very quickly.
It didn't work out that way, but there would be an internal logic to this.
Transgaz is probably more secured on the landside and blowing it up would also limit Ukraine's ability to get gas from western europe. There already were times before the invasion where Gas was pumped towards Ukraine.
The Black sea isn't a safe space to operate in, obviously.