If you're considering the nationwide Japan Rail Pass for use on the Tokaido (Tokyo-Shin-Osaka) Shinkansen, the Green Pass is extremely useful for easy access to seats on the Hikari Shinkansen. As you may know, the most frequent and fastest Shinkansen, the Nozomi, may not be used with a rail pass, so the limited-stop Hikari is the fastest service available to Japan Rail Pass holders.

The Hikari is very popular with people traveling to and from the cities of Shizuoka and Hamamatsu, which are not served by the Nozomi. Now consider the fact that there is often only one Hikari departure per hour from these cities, and the result is crowded trains. Hikari are often crowded or completely sold out in the regular cars, especially on weekends. However, the Green Car generally stays open and obtaining a seat is not too difficult, even if making a last minute reservation.

In addition to the bigger seats, the Green Pass does allow you greater flexibility in your travel plans by providing reliable access to the Hikari. If you go without the Green Pass, you risk having to sit apart on the Hikari or worse - having to stand up because you were unable to make a reservation and cannot find any seats in the non-reserved cars. This doesn't always happen, but many members of this forum can attest that standing up on the Shinkansen is an experience to avoid, and IMHO the Green Pass is the best way to do so.

All Shinkansen service has Green Cars except the Kyushu Shinkansen and the Kodama service on the Sanyo Shinkansen, both of which have very large and comfortable regular seats (2+2). On regional railways, the Green Pass can provide better access to limited express services.