They may have wanted to use the underwear, rather than a hair brush because they susupected the stains were not normal.
That if they got the DNA from the underwear accepted as being Danielle's at the PH, then at the trial can prove the stains have DNA (or are semen) from DW or DVD, then they have a locktight case.
Were they to get DNA from a hairbrush, and DNA or semen from a pair of underwear, but the underwear DID NOT HAVE DANIELLE's DNA (or they had not established that), they could not prove anything, except maybe that Damon (for instance) used a pair of Danielle's undies to wipe himself off after sex with one of his swingers.
See what I mean ?
It was Danielle's DNA in the underwear. If it had been DW's or anyone else's the DA would have been doing the Happy Dance Without Duct Tape in front of the full press contingent. That's really sort of a mute point since the body was found.