What is the 'older' accusation Socrates is faced with? Why does he feel he must engage this accusation?
The older accusation(s) facing Socrates is that he investigates things that belong to the gods. In other words, he investigates cosmological questions of what the universe is and how things in the universe work. He is also accused of teaching a rhetoric that allows his students to win over a strong argument by using a weak argument.
Two reasons Socrates chooses to address the older accusations against him are that they have prejudiced the jurists against him from years past and that, since the accusations and prejudices are so deeply seated, he can't summon his accusers before the law to question--and disprove--them.
Socrates makes an attempt in his defense against the older accusation(s) to prove a distinction between himself and (1) the Presocratic philosophic teachers who focus on cosmology and (2) the sophists who accepted large payments of money to teach young men of the city.
A critical argument Socrates makes in his defense is that, although the Pythian prophetess speaking as the Oracle at Delphi claimed him to be the wisest man known, he is not wise in himself and that his questioning examinations of noted wise men in the city of Athens--politicians, poets, scholars--was meant to discover the man or men who were wiser than he.
After meeting at time with genius in these politicians, poets and scholars, he never met with wisdom. Since both Socrates and the men examined have no wisdom, making them equal, but Socrates knows he is not wise while the others believe themselves to be wise, he, Socrates, turns out to be the wisest after all because of his self-knowledge that shows him that he is not wise at all.