With Echo we have the option of setting how we want to be notified about most events. Importantly we can choose if we want the notification delivered via web, email or both through mw-prefsection-echo. The exception to this are watchlist notifications where we can choose to have these notifications delivered as email on mw-prefsection-personal but we cannot chose to have them delivered as a web notification.
For new contributors this leads to some confusion as there is no reason to see the watchlist email notifications as any different from other notifications. Infrastructure wise it means we have to maintain two separate notification pipelines.