Users that are new to Windows 10 often ask why they should sign up for a Microsoft account. It wasn't necessary before, so some users don't see any value in the feature and choose to use Windows 10 without a Microsoft account. For those that choose to take advantage of their Microsoft account, though, it will provide them with several useful features.
Windows 10 is designed to work very closely with Microsoft accounts, so many of the built in applications use them to store data. Since it's all on one account, you only need to enter your information once and your Windows applications can do the rest. Data is synchronized between your different applications, and to use any new Microsoft services you only need to sign in.
Microsoft accounts also grant you access to Microsoft's cloud services, such as OneDrive. OneDrive gives you 15GB of free storage that you can access from anywhere, as long as you have an internet connection and your username and password. The OneDrive software automatically backs up data in any of the designated OneDrive folders to the internet so your data is always safe and accessible.
Microsoft accounts can even synchronize settings between different devices, such as another computer, a Windows phone, or an Xbox game console. You can log in to any of these devices using the same username and password, view and edit your settings, then log in anywhere else and have your settings automatically downloaded and applied for you.
Because Microsoft accounts allow for convenient and location independent access to all of your data, they are favored by users that are regularly on the go or that depend heavily on internet based services. Those with a more simple workflow might find that they have less to gain by using a Microsoft account, but no harm will come from having one.