Asking users is quite easy. Respecting their choices is the important part...
GDPR compliance has brought a lot of pain to companies dealing with European citizens' data and forced everyone to be responsible with the tracking and data gathering. And beware, some sites are simply faking compliance by asking users for consent and then ignoring it.
Sadly (and now illegally), most companies continue to drop tracking codes from various companies even if the user has expressedly denied consent for this. What happens on your site if a user refuses tracking?
There are situations where a user simply wants to avoid their data to be collected and processed by a specific company (that you happen to use on your own properties without knowing) Can your users prevent you from using specific companies on them?