Lightning Bolt: Not much needs to be said here, but I'll say it anyways. Bolt does damage and I need to do damage to win
Lightning Helix: Some Jeskai lists don't run all 4 of these, but I think they are too good not to. They can handle creatures that I shouldn't waste Path to Exile on, damage planeswalkers, and obviously damage players. The lifegain is very important, especially if I have to deal lots of damage to myself with lands or if I have a bad early start/they have a good early start. Overall fantastic card and is extremely fun to snap back as well
Path to Exile: This honestly makes the deck work. For decks like Tron and etc where the creatures are too big to bolt, this is a necessity to keep me alive. It is a non-bo with Mana Leak which is why I only run 1 of those mainboard, but I'm not too worried about it. In a pinch you can even path your own creatures to get a land, especially if your creature is dying anyways. Versatile and downright great card and an obvious inclusion in any Jeskai list
Remand: While some people have moved away from remand since it is only a temporary solution, it seems to pair well with Spell Queller when I don't initially have mana open for it. I can remand it temporarily(and draw a card, which is important so I can keep up still) then take care of it better again with Queller. It should be noted that it is bad against cast triggers, the one example I always think of is Ulamog, the Ceaseless Hunger because you don't want 4 of your things exiled. So it will occasionally get boarded out for Mana Leaks or something else that is relevant. The card draw attached to this card is what makes it good though, I need to dig through my deck quickly to keep up my game plan
Spell Snare: This is a good counterspell, plain and simple. Can be kept in on a game to game basis, it obviously gets boarded out for decks who have little or no 2 drops. But it can easily win counter wars where things like Remand and Mana Leak are flying around. Or you can counter a Tarmogoyf with it. Just a good card to have mainboard even though it is narrow in scope
Mana Leak: Only a 1-of in the mainboard due to the non-bo with Path to Exile. This card is good for when you really need something to get countered and go to the graveyard. That's why there are more in the sideboard, but no more than 1 are needed in the main for sure
Logic Knot: This is a spicy 1-of that can catch people off guard. It's rarely ever bad, but it is rarely anything more than good either. When it does work well though it is oh so satisfying. Usually one of the first things to get boarded out(There would be no point in a sideboard if there weren't cards you take out sometimes!)
Boros Charm: This card can do a lot of things for you in a pinch, whether you need to protect your permanents or squeeze out that last bit of damage you need to close out the game. Testing right now to see if this is worth it
Electrolyze: Really good against tokens, or other x/1 creatures. I often end up just using it as a burn spell, but it draws me a card while it burns which is why it is still in here. This is often the first thing to get sided out as well if they don't have any x/1 creatures