Denti
Airlines probably don’t opt for them to save money….just the way they don’t buy other stuff that would enhance safety but is not mandatory. On the A220 most buyers seem to opt for them, though.
A HUD is useful during all phases of flight as it will allow you too look outside and see flight parameters at the same time. As the depiction is “conformal” you will see where in the real world a heading leads, where the aircraft is moving, whether something is above or below you, etc. They allow for very precise flying as the depiction has a much higher resolution than a typical PFD, so even when you think you’re all over the place, an observer watching the PFD will admire your rock-steady approach. Their only real drawback is a bit less visibility in dusk/night conditions which can be overcome by looking around them. You could also argue that they spoil you for HUD-less aircraft as you get used to the simplicity of having all essential parameters in a dime-sized area. In real ops opinions are divided but I’d say less pilots hate than love them. IFALPA recommends them as a safety feature for all aircraft.