I just examined my Marlin lever rifles and found that the 1893 and the 1895 (current production) use this barrel extension "hood", whereas the 1894 (current production) does not.
The "hood" does not appear to guide the bolt at all. In fact, there is no contact. A cartridge nose will bounce off this hood during the chambering cycle. So, it may be there to increase feeding reliability during rapid cycling.
If I had to guess, a secondary function of the hood is to fill in the gap between barrel breech and where the receiver threads ends. Without this "hood", there's be a place for dirt and debris to accumulate.
Why doesn't the 1894 have this? The shorter pistol cartridges (.45 colt, .44 Magnum,.44-40, .357 spl) are much less critical in their feeding/chambering position than the longer rifle cartridges (.444, .45-70, .450, etc.
HTH
John