It IS desirable on a garand. You can use them to determing the era when the stock was produced.
Absolutely. The cartouche is the military inspector's mark. As far back as the late 19th century, the Army placed inspectors in the factories of civilian arms contractors. The inspectors would sign off on a part by making a mark on it. Both metal and wood (stock) parts would often be marked. A typical mark consists of the initials of the inspector. If multiple contractors were used, the mark would often include both a code for the contractor and the initials of the inspector. That way, you can link any part to its manufacturer and inspector by the inspector's mark. As the dates when various inspectors served is a matter of record, you can also identify the period in which in the part was produced.
If you have a USGI firearm, the presence of a correct inspector's cartouche on the stock is extremely desirable, as it indicates that stock is probably original to the gun. By "correct", I mean a cartouche that is appropriate for the manufacturer and date range of the receiver and other major parts.
For example, I have several CMP Garands. One has a Springfield Armory receiver whose serial number dates it to June of 1945. The barrel is also SA and is dated May 1945, so it's probably original to the receiver. Most of the other metal parts are also of the early 1945 period and SA manufacture, so they are likely original to the rifle (the rear sight is a post-war refit, but that's another story). The only really obvious replacement part is the stock. If it was original to the rest of the rifle, it would have a cartouche consisting of a box with SA over NFR, for General Norman F. Ramsey, the M1 inspector at Springfield Armory during the last part of WWII. Instead, I have a stock that lacks any visible inspector's cartouche, but does have RRA pressed into it. The RRA stands for Red River Arsenal (Texas), and signifies that this is a replacement stock that was fitted to the action when the rifle was rebuilt at RRA.
If my rifle had a stock with the SA/NFR cartouche on it instead of the RRA mark, it would be worth a lot more on the collector market.