I had that reaction too. The author claims that the role of long sentences is overblown, since most offenders serve relatively short sentences (and don't subsequently reoffend). But the availability of long sentences to prosecutors might convert a lot of cases that wouldn't involve custodial time into felonies.
From the paper:
The primary engine of prison growth, at least since crime began its decline
in the early 1990s, has been an increased willingness on the part of district
attorneys to file felony charges against arrestees
Part of the reason for that is legislatures have given them more options in that department. The money crimes that were created to give cops an oblique way to go after drug dealers apply to pretty much any crime, i.e. if you rob a bank it's likely they'll be able to charge with with money laundering, structuring, currency smuggling, wire fraud, or any of a dozen of those types of crime. Plus, conspiracy goes with just about everything.
Prosecutors don't have to take much to trial any more, since instead of charging you with just the crime people recognize (say, bank robbery or drug dealing), they can add up a bunch of lower level 3-5 crimes and have you looking at 300 years in jail unless you take the plea bargain. 98% of federal prosecutions never go to trial - we used to laugh at the commies for that sort of thing.
From the paper:
The primary engine of prison growth, at least since crime began its decline in the early 1990s, has been an increased willingness on the part of district attorneys to file felony charges against arrestees