Influencing DA elections is powerful aside from the obvious impact on the normal criminal justice case load, especially since it also:

(1) Creates a deep bench of politically connected and experienced lawyers for later promotion to judge or federal position, where policy is really made (cf, The Federalist Society – the smartest and most effective thing non-progressives have done in the last 35 years.)

(2) Allows for the influence force-multiplier effect of free, flood-the-zone media coverage (cf Trump) of politicized show trials that ordinary prosecutors wouldn’t pursue for likely failure. Consider the Zimmerman trial, or the fiasco with the Baltimore cops with the added ‘benefit’ of the potential for mass unrest.

(3) Settlement / Consent Decree abuse (a huge and under-reported problem in the law, related to Plea-bargaining abuse). If states don’t pass equivalents to Goodlatte’s “Stop Settlement Slush Funds Act” (which of course doesn’t even yet exist at the federal level, so good luck in Blue states), then local DA’s can essentially shake down every deep-pocketed enemy entity in their jurisdiction and divert the money to their nonprofit political allies who will, in turn, launder it through the incomes of well-compensated administrators and kick back some of it to campaign funds. Local DA’s can also arrange for pretext settlements wherein other local governmental entities need an ‘our hands are tied’ alibi for implementing politically unpopular policies they already want to do.