This may seem a little strange, but quite a few riflemen become upset at the very concept of collimators. I've run into several who've gotten really huffy at the mere mention of "bore-sighter," asking why they should spend good money on one, when they've never had any trouble sighting-in a rifle.
This makes sense as far as it goes, but the several bore-sighters on my workbench are primarily used for aligning scopes with the bore during mounting. When huffy shooters are questioned about "never had any trouble sighting-in a rifle," most admit they've run into scopes with inaccurate adjustments, or that "ran out of adjustment" so couldn't be sighted-in.
While some scopes have clicks as unreliable as 7-day weather forecasts, often inaccurate adjustments result from scopes not being reasonably aligned with the barrel. The point of impact in internally-adjusted scopes is changed by turning screws contacting the erector tube inside the scope. The adjustments in typical 1-inch scopes work most accurately when the screws contact the middle of the erector tube, instead of off to the side--as they do when a scope's misaligned with the bore.
The easiest way to prevent this misalignment is by using a collimator when mounting the scope. Now, the same job can be accomplished by classic bore-sighting-- looking through the barrel while pointing it at some small object, then seeing if the mounted scope lines up with the object. But in 40-some years of mounting scopes, exactly one of my workshops had a window where rifles could be bore-sighted easily, using the peak of my neighbor's house across the street. This might be a feature to consider when building a workshop, but might be more expensive than buying a collimator--and doesn't work with rifles where you can't look through the bore, such as lever-actions.
A few shooters don't like collimators because some counter-jockey in a sporting goods store used one on a scope they'd just purchased, and the first shot didn't even land on a 100-yard target. This can happen, but no pre-shooting alignment technique guarantees the first shot will hit a foot-square target, because bullets don't always fly exactly where the barrel points.
Oh, once in a while they do, but normally different kinds of ammo shoot to varying spots on a foot-square target--or miss it completely when a rifle's sighted-in with another brand. Consequently, the first thing I do with a new collimator (manufacturers sometimes send me new models to...