The rollsizing process removes the bulge in the base of a fired case to bring the case back a Mid SAAMI dimension. When combined with the normal sizing dies the case will be very close to the SAAMI specifications. This means you can now get Factory grade reloads with a standard reloading press. Reliability and consistency wins matches.

The full-length sizing die cannot reach the bulged base of fired case. When the cases are rollsized and then reloaded as normal, the round will feed and chamber reliably. The amount of blow out in a case depends mainly on the chamber in which the case was fired, the powder charge and recoil system used. The most common cause is when cases have been used in carry or LE firearms with unsupported or looser chambers due to the prerequisite of reliability over accuracy. The brass from these guns can often be re-used but without rollsizing the loaded round will not reliably feed or chamber in your firearm, escpecially if it is a tighter competition / target application.

Very hard to say, generally much longer than without rollsizing, but too many issues contribute so it is too hard to state exactly. The designer of the rollsizers gets over 40 reloads with rollsizing with regular starline / federal / Winchester brass. There is no evidence of fatigue in the brass with rollsizing from any of the customers who reuse their brass.

Yes, all rollsizing processes require cases to be resized after every firing. Failure to rollsize after each use will increase the likely hood of feeding and case reliability. The resizing process is usually part of the reloading process, so no additional effort is required.

Ammunition is made to a specification known as the SAAMI Specifications (Sporting Arms and Ammunition Manufacturers Institute). This is the rulebook covering ammunition and corresponding chamber dimensions for all ammunition manufacture, and all the major OEM manufacturers follow this. The Factory ammunition is rarely (if ever) loaded to the maximum size, the industry standard is about mid way between the maximum and minimum dimensions but the vast majority of online dimensions  only show the Maximum dimensions. The Factory dimensions are generally speaking the most consistent and reliable. Too large and the ammunition does not feed / go into battery. Too small and the magazines do no work, and the ammunition does not seal within the chamber. It’s all about manufacturing tolerances. 

This becomes even more important for those wanting to use reloads for competition or accuracy based sports (target shooting, metalic Silhouette or IPSC / USPSA etc.). Guns used in these applications usually have tighter tolerances especially in chamber dimensions and reloads in these applications are very difficult / impossible to reliably achieve without rollsizing.