[jamesmaniscalco]

As an accelerator physicist - while some earlier machines were built without vacuum beamlines, nearly all modern[0] accelerators are built with ultra-high vacuum beamlines. Beam quality greatly degrades in contact with residual gas due to scattering. This is especially important in light sources, but really any machine where you want low emittance.

[0] edit - I should specify that I mean high energy machines here (my focus). There are also many low energy machines that don't require high vacuum at the end - medical accelerators for example.