bevel/vernier protractor use

I need to pick up a bevel/vernier protractor of eBay but have not used one before. Watching some explanations on youtube it looks quite straight forward but one thing that has got me a little confused is if it can read angles below 90 degrees without the attachment.

Most of the used ones on eBay seem to be missing this part and it’ll cost me more money and or time to find one with it. Looking at the thing it looks like it would read an angle below 90 degrees just fine..
Attachment 28431
This is a still from a video where he says just after this you need the attachment for less than 90 degrees?

In every introduction video or description I’ve read, they always seem to add the note that this part is for reading angles less than 90 degrees without clarifying whether it's needed or just assists in reading them.This is the part I'm talking about:
Attachment 28432

Does anyone know how to use one and if this is true?

http://site.iugaza.edu.ps/aabuzarifa...O20152_CH4.pdf