How can they not effect the sound

A small dent in a speaker cone usually doesn't significantly affect sound quality because the cone is designed to flex and vibrate across its entire surface, meaning a localized dent doesn't drastically disrupt its movement, especially if it's only on the dust cap which is primarily a protective layer; the key factor is that the cone material remains intact and can still move freely to produce sound waves. 

Key points to consider: 

• Dust cap function:

  The part that usually gets dented is the "dust cap," which sits on the center of the cone and is mainly there to protect the voice coil from dust - not to contribute significantly to sound production. 

• Cone material properties:

  Speaker cones are designed with materials that can flex and vibrate evenly, so a small dent might not disrupt this movement enough to cause a noticeable sound quality change. 

• Severity of damage:

  While a small dent might be negligible, large dents or tears in the cone can significantly affect sound quality by disrupting the cone's ability to move properly. 

I used to own some wonderful Von Schweikert speakers.  The dome tweeter of one was dented.  On the advice of VS, I used the tube extension of a vacuum cleaner to remove the dent.  The directions were as follows:  with vacuum on and tube held steady, pointing straight at the dome, slowly approach the dome. When you get very close without touching, say an inch or two, it will pop out.  I did and it did.  

Depends where. For a dome tweeter a dent can do a lot of bad things.

The use of a vacuum is a great fix. Also, with some tweeters the dome is removable and you can fix them by pressing from behind. This is the preferred solution with tweeters, as suction in a small area may be hard to control without excess force in the suspension.

Tweeters don't have a "spider" holding the dome in place.  The surround is the only part of the suspension and it comes out when you disassemble the face plate, when manufactured to do so.  That is, with the right screwdriver or Allen wrench the whole thing comes apart and gives you access to the back of the dome.  Not true for mids and woofers I'm afraid. 

That is, with the right screwdriver or Allen wrench the whole thing comes apart and gives you access to the back of the dome.

...and occasionally to the ferrofluid hosted in the magnetic gap, or what's left of it.

From my experience, dents may be in the dustcap in the middle of the bass driver in which case, unless the damage is extensive, it is likely no effect on the sound.

My first task in my first hifi job at a Tech Hifi that specialized in refurb gear, was testing used speakers prior to attempting sale. Those with voice coil issues, usually due to being driven by amp that was clipping, had to have drivers replaced prior to sale.

In many cases, the issue was merely a pushed in paper dustcap. Ugly and a defect for sure but no impact to sound. Solution: replace the dustcap, or the easy fix: use a safet pin to punch a small hole in teh dustcap and push it back out to normal from behind.

THese speakers were then sold at a discount determined by magnitude of defects, if any. Common practice to this day.

Dented soft dome or other material tweeters: different story. Any physical defect there is likely to have an effect on the high frequency sound. Could range from minor/not a significant issue to major. Devil will be in teh details.

From experience I learned never buy expensive speakers with exposed soft dome tweeters unless there is a grill or some other way to insure no accidents. If you have young children in the house: doubly true.