Longer length siding is typically offered in 16', 20' and 25' ranges. The primary benefit of longer length siding panels is that it reduces the number of seams on the wall. Siding seams can be aesthetically unpleasing. With longer length panels, there are fewer seams. For example, on a 20' wall, a 16' panel creates 35% fewer seams than a standard size panel, and a 20' panel reduces seams by 100%. In short, the longer the panel the fewer seams.
Longer lengths can also reduce the amount of time required to install siding since more area can be covered with fewer panels. Most longer length sidings are designed to be installed with their standard-size counterparts so that various sizes can be used together on a project for efficient utilization of material.
Your siding contractor can determine whether regular length or long length siding will be better on your home (or both together), and they should have experience installing both siding lengths.