I think this done to create a slight overpressure which helps keep the dust out.

Your question isn't very clear... Is it why do they use computer fans? (My guess is that is what they had). Or is it why do you blow instead of suck?

You want to suck air through the filters rather than blow into them. For one, this keeps the fans cleaner but also it's more efficient, reducing turbulence inside the box.

https://old.reddit.com/r/crboxes/ is a good resource if you're looking to make your own. There's been a general shift from large box fans to PC fans because their performance/noise ratio is better.

I think it's because there are no grills on the outside. If the fans were sucking air out of the box, dust would build up on the outside, and bumping it would dislodge dust back into the environment.

With the fans blowing in, all the dust is on the inside of the box (and on the fans).

The box fan version also blows air into the box