Highlighting and note taking is available at your finger tips and does not require additional tools which might break your flow.
When using a dedicated eReader device (ex: a Kindle), you are locked into an focused [[!ecosystem]] which can help minimise distractions within the environment (though not necessarily from without). Physical books are even better at this in some ways though not others (see highlighting point).
Anecdotally, I can read an essay on my Kindle without distractions for far longer than an lengthy article or PDF on a laptop or phone.