Roller bearings should be greased if they are not sealed bearings.
If you have sealed roller bearings they are greased from the factory and require no further greasing.
However if the roller bearings are not sealed then they will require greasing to keep them in good condition.
The grease in the roller bearings helps reduce friction and reduces wear and also protects the surfaces of the roller bearings against corrosion and extends the bearings lifespan.
Greasing bearings that require grease including some roller bearings is crucial to ensure they don't prematurely wear out and seize up.
Without grease the roller bearings and other bearings would eventually seize up and could even break the shaft it's on.
The grease reduces the friction between the internal components and prevents metal to metal contact and ensures smooth movement.
Some roller bearings may require oil and others require grease as it depends on the roller bearing and what the roller bearing is on.
Some roller bearings are pre greased at the factory such as those used in some vehicle wheel bearing hubs or in industrial machinery.
In those cases the roller bearings don't require any additional greasing.
If there is a grease port then it requires grease and if there's an oil port then the roller bearing requires oil.
Increased friction, heat generation and accelerated wear can result from not enough or not any grease at all.