There are a great many things free on DTRPG that I grab solely because they are free; it there were a limited budget, I'd not get them. Let's take the preview of Blood & Doom... I only grabbed it because it was free, and only noticed it because of a cat walking on the keyboard and adding it to cart. (Trackpad is on the keyboard.) the 5 minutes spent skimming? well worth the time to DL it as a free product, and plenty to find it not worth me participating in the KS.
If I'm browsing DTRPG, I often grab a freebie or two, and have done so since about the turn of the century. my digital horde is hundreds of "purchases" and several thousands of items...
Most of which get skimmed once and filed. The better ones get reread and/or played.
There is, however, a lot of stuff that I'd never have even looked at save for it being free. Much more that was bought as part of bundles, where i bought the bundle because it was cheaper than buying the one book in it I really wanted. (WFRP 2E, for example, had the first legal PDF of the WFRP1E PDF. 7th sea 2E had all of 1E in it, and I wanted to read the rest of 1e...)
Free item sales do affect total orders, and the proportion of PWYW is increasing from what it was. Especially since an order of nothing but free content is still an order number.
Rob's excellent tracking is, however, lacking a bunch of details.
It's probably not the actual DTRPG numbers; my Wargames Vault library is the same 539 orders and 1500+ entries as my DTRPG one. While I don't have any Drive Thru Comics nor Drive Thru Cards orders, I suspect the number Rob is tracking is total OBS sales across storefronts, not just DTRPG.
(and my first showing order? I paid $8 but it doesn't show what... order number just shy of #150,000 No product listed in it! 23 jan 2003)