While it costs 5 mana, that's not too much for the Blue/Red counter-burn decks that wanted to play it in Standard to manage. Plus, it can tap for red mana as soon as you play it. So, Pyromancer's Goggles essentially only costs 4.
With cards like Magmatic Insight, Fiery Impulse, and Exquisite Firecraft in Magic Origins, there were plenty of options for spells to copy with Goggles during its time in Standard. Craters' Claws was another good card to copy - since its X cost is also being copied, allowing you to double down on damage. Pyromancer's Goggles made cards that were only OK before and turned them into powerhouses.
The draw cards Magmatic Insight from Magic Origins and Tormenting Voice from Dragons of Tarkir both have the requirement to discard a card (specifically a land card in the case of Magmatic Insight). However, by copying them, you're only discarding one card to draw four in return. This is because you only cast the card once, so you only discard once since the cost is actually only paid once.
Keranos, Lord of Storms was a great card to play alongside the Goggles in Standard. Keranos gives you card advantage and the potential to deal 3 damage on every one of your turns. The under-appreciated counter-burn spell Mindswipe was good to play in this sort of deck, too - plus the always useful Dissolve. With extremely efficient burn spell Stoke the Flames still in Standard at the time, this sort of deck could do some real damage.
Another card that could combo well with this card is one that LSV suggested in his preview of this card, Invoke the Firemind, which is in Modern. Essentially, you could choose to burn your opponent and draw cards, burn your opponent twice, or draw cards twice. That's hot stuff and well worth the original 5 mana investment in this card. Plus in Modern, you have Lightning Bolt, Lava Spike, and other powerful burn spells to copy. Aurelia's Fury, Lightning Helix, Boros Charm, and Atarka's Command work, as well, as they have red in their mana costs.
You could even copy an Epic Experiment and play a whole bunch of spells for basically free. That sort of combo deck has been done befor. Having the Goggles only makes it more consistent. Unfortunately, with all of the potential good combinations with Pyromancer's Goggles in Modern, being a 5-mana artifact in such a fast-moving format means that a Goggles-driven Modern deck hasn't emerged on the competitive scene as of 2019.
However, Pyromancer's Goggles has found quite a few homes in EDH. Commanders that play the Goggles include Chandra, Fire of Kaladesh, Neheb, the Eternal, Firesong and Sunspeaker, and Wort, the Raidmother, all decks that use substantial numbers of red instant spells. Pyromancer's Goggles has also been tried in some builds of Zada, Hedron Grinder, as it provides some redunancy for copying your instant spells, but it hasn't become a staple of the deck.
One Commander that hasn't really used Pyromancer's Goggles is Feather, the Redeemed. But, that's partly because Feather is Red & White and the Goggles don't help non-Red spells. Were you to build a pre-dominantly Red themed version of the deck, however, Pyromancer's Goggles is worth considering.
The other good thing about Pyromancer's Goggles is that the Core Set 2020 brought with it a mini-Chandra theme. That's good because these include THREE Chandra planeswalkers, Chandra's Regulator (which copies Chandra planeswalker abilities and also has a draw engine built into it), and Repeated Reverberation (which copies an instant, sorcery, or loyalty ability TWICE). So, if you're wanting to build a Red Spellslinger deck thanks to this card, you will want Pyromancer's Goggles.
There are a bunch of things you can do with Pyromancer's Goggles, and you really only need 1 or 2 copies in a deck, since it's Legendary anyway. The pieces are definitely there for it to be even more popular in EDH than it already is, although it's proven too slow for Modern so far. Of course, Pyromancer's Goggles has plenty of casual appeal, as well, and it's well-deserved.