Newbie ??? about Tubeless Tires
 

New to mountain biking and new tubeless tires. I'm a little confused. It's my understanding that if a tubeless tire is punctured, the sealant inside the tire will seal the puncture and that if the puncture is so bad that it won't seal then you're gonna need a new tire anyway. So, why is it that every "essential" mountain bike gear list I look up has tubeless puncture repair kits and pumps on the list?

I feel a little naive asking this question but I would like a little clarity beyond sponsored lists designed to sell products.

Some punctures are big enough to keep leaking but small enough to seal with some help.

When I was deciding whether to go tubeless I surfed around forums for stories of people having failures. Most of them were either dried out sealant, or rim and tire damage so severe that a tube would not have helped either. There was a persistent problem earlier on of pinch flats and burps, but most tires now have armored sidewalls and/or some people run Cush Core or similar products, especially for DH or enduro. I decided for my riding, which is mostly short hauls in well trafficked areas, I'd just carry sealant. I have a tube now but Darts are on my Christmas list and might replace it. If I were going to Downieville or Moab or something I'd carry more repair stuff.

See for example this (promotional and likely prejudicial) demo by Worldwide Cyclery of the Stan's Dart:

There can be punctures which are too large for the sealant to seal, but can be repaired.

There is always a chance you could be walking home. Carry a tube with you along with your plug kit. Bring a rag to wipe your hands with also, because putting a tube in a previous tubeless setup is MESSY.

Flats don’t lend themselves to such binary classification.
They run the whole range from self-fixing (with sealant) to fixable with assistance to entirely beyond repair.
MTBers can end up a fair distance away from the nearest road, which tends to promote considerable interest in being able to ride out even after encountering mechanical issues. So anything that’s easy to carry, and increases your repair ability has a given customer base.

When I did a ghetto-tubeless conversion on my winter wheels, I discovered that one of the tires leaked through the hole left by a missing stud. And sealant alone wouldn’t plug it. So I made a small tuft of that fibrous stuff that can be used to seal plumbing threads and poked it in there, and through the tire. Within minutes, the leak stopped.

With a tubeless tire, sealant is the first line of defense for small punctures. If the puncture is too big for the sealant, you can probably plug it with a simple, small, cheap plug kit. Beyond that, any slice big enough will require that you glue a patch inside the tire. This size hole would be big enough for a blowout with a tube anyway. At that point, If you have too far a distance to walk, you'd best just boot the tire inside and out a tube in it. This is exceedingly rare. When I pull a tire off the rim, there are dozens of little boogers from where the sealant plugged small holes.

I've been riding tubeless for about a decade and the frequency of flats that cause me to stop are about 2–3 times a year. That's on Texas terrain where cactus, mesquite, and sharp limestone ledges cause the stragglers using tubes to stop 2-3 times per ride. I top off my sealant every few months and carry a pump, CO², tube, and plugs. I've gone through a few tubes in the past few years and only one was for my own bike. The rest were for bailing out unprepared riders whose tubes had failed them.

What I've always wondered about the tube was how would that work with all the old sealant in my tires. I recently repaired the sidewall on one of my tires (including some stitching up!) and there is no way a tube would have worked well; it was so sticky in there I could barely keep the tire from sticking on itself. Thinking out loud though, Maybe the tube inflating would actually help that situation.

Anyways - OP, you are right - you need to carry a tube or a tire plug kit along with a pump to get back going if you have some trouble. You should bring a side wall patch as well (park tools sells them).

In this video Eric Porter does a tube repair with his kid, who had a pinch flat when he landed hard on a concrete edge (at 4:11)

Since apparently I'm just watching old videos all weekend, here's Eric Porter again with a bacon strip repair (at 5:25)