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 Par'oh to see Hashem's strength?
Seforno: So that he should do Teshuvah.
Seforno: Because Hashem does not want the Rasha to die - Yechezkel, 18:32. According to the opinion that Par'oh survived K'ri'as Yam-Suf and became king of Ninveh, he did indeed do Teshuvah.
What did Hashem mean when He added "u'Lema'an Saper Sh'mi..."?
Seforno: He meant that Par'oh should tell the world, and prevent many people from sinning.
See 9:16:1:1** - In which case he also saved the people of Ninveh from sinning further - by causing them to do Teshuvah).