If the video wasn't obvious enough, let me break it down for you.
1. Finding criss cross lashes
This one is super important. The difference from a messy set to a neat and tidy one is having all of your lash extensions in the same direction. If you have some that are crossing over or are on an angle, it will make your set look messy, and in return...sloppy.
2. Finding layers
If you haven't noticed, everyone has different layers to their natural lashes. A top layer, middle layers, and a bottom layer. By knowing the difference between layers you can then decide which curl/length you will place. If you're just lashing willy nilly and not taking layers into consideration, again your lash set will be messy. You shouldn't be applying the same length on the top layers as well as the bottom. Using your mirror will make it easier to see, rather than lifting your clients lid up.
3. Finding debris
Sometimes debris can be sneaky and stick in hard to find places. By using your mirror you're able to see all the lashes at all angles and find anything that may be hiding. It's also a good chance to snap a photo at that angle to show your client and educate them.
4. General comfort
Like I said above, sometimes it's hard to get a certain angles while you're lashing. It's better to use a mirror than to lift your clients eyelid. They'll have less sensitivities to the fumes and they'll be more comfortable.