The ability to synch your Messages (as well as numerous other services) across multiple Apple devices is undoubtably a huge convenience, but also one that could lead to a serious breach of privacy. Obviously nobody wants their private messages accessible to friends, family and co-workers. If you are the type to have your text messages arrive on every Mac that you use, you might want to check which ones you are logged into, as you wouldn't want your messages popping up on a Mac that you share with a roommate, or on an iPad that you left at a friend's house.
Taking stock of monthly subscriptions now and then can save money. After all, why pay for something that's no longer being used? iCloud Drive usage varies widely depending on how the service is being used. If your iCloud subscription exceeds your needs, it's simple to downgrade and reduce the monthly iTunes charges.
The out-of-office reply is a handy feature that lets you respond to emails automatically with a message saying that you are out of the office, or anything else you want it to say. Just about every email provider offers this feature, though Apple's Mail app does not have the option - you have to set it up on your Mac with Mail rules, and they are client-side, which means your Mac has to be on the whole time you are away in order to respond with your message.
There are many apps that will manage your email accounts besides the iOS Mail app - Thunderbird, Outlook and Gmail will all do the job, to name a few. Outlook is popular with a lot of businesses, so if your work email is managed by Outlook you might want to add your other emails just to be able to access them all in one place. Or maybe you prefer the Outlook interface to that of Mail.