Inside Paris, a car is more trouble than help: the métro is faster, parking is scarce and expensive, and the city is a low-emission zone. Where a car makes sense is the day after, when you leave the city.
For day trips outside the city: Versailles is about 20 km away, Giverny about 75 km, Reims and the Champagne vineyards about 145 km, the Loire castles around Chambord about 180 km. Trains reach several of these, but a car lets you combine stops.
For the rest of a trip through France, once you have left Paris.
Pick the car up on the day you leave Paris, not the day you arrive. Airport and outer-station rental offices save you driving through the centre.
Paris is a low-emission zone: rental cars carry the Crit’Air sticker it requires, but check it if you drive a car from elsewhere.