What was Hashem referring to when He said "Lema'an Har'oscha Es Kochi"?
Rashi (in Yeshayah)
: He was referring to K'ri'as Yam-Sof, where Egypt witnessed Hashem's strength in full force.
Why was it important for Pharaoh to see Hashem's strength?
Seforno: So that he should do Teshuvah.
Because Hashem does not want the Rasha to die (Yechezkel, 18:32 [Seforno]). According to the opinion that he survived K'ri'as Yam-Suf and became king of Ninveh, he did indeed do Teshuvah.
What Hashem mean when He added "u'Lema'an Saper Shmi..."?
Seforno: He meant that Pharaoh should tell the world, and prevent many people from sinning.
See 9:16:1:1** - and he also saved the people of Ninveh from sinning (i.e. he caused them to do Teshuvah).