There are a couple of benefits to using a notched squeegee, but they may not benefit the DIYer in all cases.
Epoxies and other coatings can be quite viscous, and pushing around that thick material can be tiring, using a notched squeegee you can just push down and since some of the coating is slipping through the notches, you can muscle around the coatings easier. Now if you're not doing a large area, this isn't as much of a concern, but if you're looking at 1,000 ft2 the easier spreading begins to look good.
All coating come with specs for the coverage, X gallons spread over Y square feet. If you have a 500 ft2 garage, and the specs call for 1 gallon per 250 ft2, it's pretty easy to split the garage into 2 sections and alllocate 1 gallon per section. Roll it out evenly and you know that you have the right amount of coverage. Now on a larger floor, and bigger projects, it can be time consuiming and cumbersome to divide the space into sections. And for contractors who have one guy mixing and running out the material, and 2 or 3 guys spreading and back rolling, it can be tough for the guys on the floor to know if they're putting down the right amount of material. A notched squeegee alleviates those problems by metering out the correct amount.
Of course some of the old timers can do the same with just a flat squeegee. Tell them you want the coating spread 12 mils thick, and they can put it down plus or minus a mil. Of course that takes a bit of experience.
You'll also notice that most squeegees you can find are for 10 mils and above. The notches for any thing less are just too small and fine to work well. Also squeegees do wear down, so you need to adjust the pressure, otherwise as the teeth wear down, and the notches get smaller, you end up with too little coating getting through.
Finally - isn't "squeegee" one of the odder words in our lexicon? I really enjoy this opportunity to use it repeatedly.