Maybe you prefer the software you have on your computer but want to use your iPad as a pen display. Astropad lets you do it.
A traditional pen display is a costly device. You may already own an iPad. Astropad connects your iPad wirelessly or via USB to your computer and lets you use the iPad as a pen display. Everything that happens on your computer display is visible on your iPad and everything you do on your iPad is affecting what happens on the computer screen.
Open Adobe Photoshop, Illustrator, Pixelmator, Lightroom, Affinity, Manga Studio, or any other Mac app, and use the iPad to paint, draw, retouch, write or whatever you want to do. The connection is very fast and you are unlikely to experience any lag.
It supports styluses from Apple, Adobe, Adonit, Wacom and a couple of others.
Astropad comes in two flavors. One (Astropad Standard) is pay (30 USD) once and use forever. The other one (Astropad Studio) is free to download but comes with a subscription (8 USD/month or 65 USD/year). Astropad Studio has custom gestures, keyboard support, settings sync between devices, priority support and monthly updates with new features. If you are a professional creative you may prefer Astropad Studio. Otherwise Astropad Standard is probably enough.