Why does Team Roket want Ash's Pikachu?

Question

In the first episode, Team Rocket saw Pikachu using thunderbolt in alot of Spearows. Since then, they tried to give Ash's Pikachu to Giovanni because he is powerfull, but when pikachu used that attack, it was raining (and I think that's what made it powerfull) and he almost fainted after using it.

Answer 1

Team Rocket did NOT see Pikachu in the first episode. I just watched the first 5 episodes, and here's the simple version of what really happened:
In episode 2, Team Rocket came to the Pokemon Center to steal the Pokemon there. To quote Jessie when Ash tried to protect Pikachu, 'We don't want your puny electric rodent.'
They were there for other Pokemon.
Later during the battle, Pikachu asked Ash to give him a power boost, which Ash did. Pikachu then used his Electricity to blow up the whole building.
Right after, Team Rocket was impressed by Pikachu's power, and swore to try and catch it.