if one can give a deposition in the form of an affidavit then why would someone ever want to appear in court to give a live testimony?

Your assumption is incorrect. One cannot give a deposition by affidavit. A deposition does provide for cross examination. Even if one is deposed and there is cross examination they can be compelled to testify at trial (by a notice to appear or supoena) so the tryer of fact (judge or jury) can evaluate the credibility of the witness.