I like to have an inline barrel adjuster on the rear brake cable on drop-bar bikes with Aero lever cable routing where no other adjuster is available. If it is put just a few inches past where the housing breaks out of the bar tape it acts as a convenient disassembly point if the bars ever need to be removed and protects the housing from being damaged and requiring the bar tape to be removed to add new housing during the repair.

With modern canti, U-brake, or old-school centerpull calipers you really must have an adjustment point and modern levers often don't have one. I always put a headset-mounted brake stop with an adjuster but the rear brake usually requires an inline adjuster if you need to add one unless the frame is equipped with a brake bridge with an adjustable barrel in it.

Be sure not to use shift-cable inline adjusters which might not be up to the strain of brake use and haven't been approved for this purpose.

Shimano and SRAM both sell brake-rated inline adjusters like this. Jagwire sells a 5mm adjuster but no longer is advertising them as fit for brake use. Use at your own risk.