All of the answers go over the protocols, but to answer your question specifically: they ask so they know if there is (also) a medical emergency and how severe and urgent the emergency is. Protocol might already mean that the ambulance has been dispatched before you are even asked that question, but the answer is still important as it can change how the ambulance responds to the situation.
So if you say yes they can then ask e.g. how many people are injured, what kind of injuries, where these people are located, what kind of people are injured (children, elderly, etc.), with this information they can then decide whether they should be sending more ambulances, if the ambulances are going with lights and/or sirens on, in some countries there are even ambulances for specific emergencies that they could be sending, and by relaying this information to the paramedics they know what they can expect and can also start preparing for the situation.
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