If you see a theme that looks exactly right for you, go for it.
However, after a time you may want a theme that is flexible enough for you to tweek and change to suit the website you are publishing.
Then you need to look for a theme that allows multiple widgets and has many formating options.
You may not want or have room for the standard widgets (Calendar, etc.)
Some themes allow your own logo, your own favicon, your own colours, and your own header image. Some allow you to tweek the fonts. Some give a choice of layouts.
You will enjoy personalising a WordPress website.