The idea is to fill the gaps but not the tapers. If you have very narrow gaps, or no gaps, there is no need to prefill. Narrow gaps will typically fill with the taping mud. Some guys advise hanging with bigger gaps or making the gaps bigger with a knife and prefilling everything.
The same but different animal... ;~) The banjo holds tape and mud and is carried to the joint and moved along the joint to apply the mudded tape. Whereas with the TapeBuddy holds the tape and mud and sits on top of a joint compound bucket (has notches to fit on the bucket) and you pull out a length of tape and cut it to length (corner of a joint compound knife) and then move the mudded tape to the joint and apply it. Generally you don't pull out more than two arms length... multiple pieces to do a long flat joint is no problem and butt joints are never more and 4.5 feet (unless perhaps you stuff a narrow piece of rock between two horizontal sheets!). With the TapeBuddy you are only carrying around the mudded tape and not a bunch of tape, mud and steel (or aluminum) so it's less fatiguing to use for mere mortals.
The host of these videos, Ben, is amusing and irreverent and knows what he's doing even if he's goofing around for the video. The video on the TapeBuddy also mentions prefilling and opening up the gaps to do so...