Because it allows the shooter to quickly and safely top off the magazine without taking the rifle out of battery.
If 'safety' is a concern, tube fed LG's are FAR safer then having a gate.
If 'speed' is a concern, go buy a semi-auto and a bunch of magazines.
As Ive stated already they both have advantages and disadvantages, but are basically equal in terms of actual use. People have these 'Soldier of Fortune' fantasies about reloading under fire....pffft. The odds of such happening are infinitesimal at best and I would never even remotely consider that as a determining factor in choosing an LG, its frankly absurd.
For a typical shooter, tube fed mags are
always easier to load, and often just as fast then a gate. Ive never seen anyone fumble loading a tube, but Ive seen highly experienced shooters with highly tuned guns fumble with a gate. Ive seen rounds get damaged and jammed in a gate as well as fair number of bloody fingers.
My favorite lever guns are the Marlin 1894's which do have a gate, but if Henry made true stainless LG's I would be just as happy with their tube fed models. In fact possibly happier as Rem-lins quality went down the crapper some time ago, although I am waiting for more reviews on their recent re-release of the 1894's.
Bottom line, pick which ever you like best overall, loading style should be the least concern.