I agree, squinting down the barrel is all you need to set up a 'scope. If you are methodical and your 'scope turrets are correctly calibrated it shouldn't take many shots to complete the process. Use a ruler to measure how far out each shot is and move the turret the calculated amount to correct the error. Don't just twiddle them randomly, trying to "home in" on the zero.
But I bought a second hand collimator at a good price and now find it quite useful.
If you like to swap scopes around, or use e.g. Weaver bases to swap scopes with NV etc. or to remove them to make space in the cabinet, then a quick check with the collimator will confirm that the 'scope is still zeroed after re-fitting.
When you have zeroed the rifle, fit the collimator and make a note of where the crosshairs fall on the grid. It won't be exactly in the middle, but it is quite repeatable.
If you change scopes, use the same setting on the new scope and it will get you pretty close. The grid on my collimator is 4" at 100 yards, and IME will get you closer than that if you note the position inside the square.
Also useful to check zero hasn't shifted after a knock etc.
Also useful to test that the adjustments on a scope are reliable. You can wind the turrets across their range and check they move smoothly and repeatably. Also wind the zoom up and down and check POI doesn't shift.
I've rejected a couple of 'scopes now with these sort of problems which could not otherwise have been easily discovered, and the collimator has more than paid for itself, and given me confidence that the scopes I have are in good nick.
This is the one I have. Other brands are available but they mostly look as if they were made in the same factory...
Opticswarehouse Boresighters