Traditional fare alerts like those offered by OTAs only track a specific itinerary from one city to another (or up to seven departure and arrival airports in the case of Google Flights) and you’ll need to choose specific travel dates.
This means if you just want to go anywhere in Europe anytime in the spring, you’d need to set hundreds of fare alerts to cover all the possibilities. Even if you have specific dates, if you were to set alerts from your home airport to a dozen airports in Europe, you could still miss an amazing deal to a nearby airport if it wasn’t covered by one of your alerts.
Additionally, the alerts aren’t instantaneous. They’re typically sent on a bit of a delay, and these alerts rarely catch a Mistake Fare, which is when an airline accidentally discounts a flight by significantly more than they mean to (and the error is typically corrected within a few hours).