iam sure there must be a good reason for this design, but the only thing l can think of is that it keeps the frost off the battery in the winter, if this is the case why not put it under the bonet & fit a lagging jacket (anti frost case) around the battery to keep the frost off,

look under the hood.no room

Maybe they forgot the room for a battery when designing the car, and putting it under the seat was the best option.

l dunno but l do know that the fiat punto has the same.

Makes more space under the bonnet, the Citroen Picasso is the same.

Because the Scénic is bonnet is quite short & the windscreen quite far forward, a lot of the engine bay is not easily accessible.
Renault have therefore moved something that is relatively easy to locate elsewhere (the battery) to leave the accessible space under the bonnet for other service items (air filter, fuel filter, etc.) that cannot be relocated to outside the engine bay.

Some other manufacturers locate the battery at the rear for other reasons - for example BMW locate it at the rear to aid achieving an even front-to-rear weight distribution.