Every new install of WordPress comes with a number of themes.
But often people install their own theme and wonder if these default themes can be safely removed.
The short answer is – YES you can delete the default themes — BUT make sure you always have a SECOND theme.
Why do you need a second theme?
If a coding error occurs in your active theme (or if it’s deleted from the server) WordPress will switch to another installed theme.
Without the second theme installed WordPress will return a white screen to your users – the “white screen of death”.
So you don’t need these themes installed, but you really should have a second theme installed – even if it’s a simple theme like Underscores.
What I find annoying is how the new yearly theme is forced onto people when it’s released. That is, when Twenty Eighteen is released it will be pushed into all existing WordPress installations. Not so bad, but still a form of bloatware you either need to manually delete or keep manually updating.