I don't want to sound condescending, and it is certainly not my intention to offend, but if your players are asking what gold is for, then you're not being strict enough.
I'm not playing 5E, so I can't speak to how that edition handles things, but...
I am currently running an epic (3.0 Ed) game, where the players have ALL KINDS of expenses:
(Yes, I realize low-level characters will not have many of these expenses, but there's still a lot of stuff for them to pay for, as well.)
There are 5 players in my epic game, hence their group name, "The Fearless Five."
Their lifestyle (3 good meals per day) costs them a lot of money. (a good meal is defined as a meal that features meat, and a good drink, such as wine, beer, coffee, or tea.) That comes to 5 gold per player per day. That means that the group pays 750 gp. per month for that lifestyle. That in and of itself is a FORTUNE in a world with a medieval-style economy. And that's just the tip of the iceberg.
I don't remember all of the following costs off the top of my head, but here's ROUGHLY what they pay for:
They jointly own a castle (formerly, Maure Castle) and they pay upkeep for it.
Two of the five have armies that they maintain.
One of the five has a monastery, and pays upkeep for it.
One of the five has a small fort, and pays upkeep for it.
One of the five has a MAJOR temple in the City of Greyhawk, and pays upkeep for it.
One of the five has two ships (a sloop, and a barque) that he pays upkeep and docking fees for.
The guy with the ships also pays a skeleton crew to live on the sloop, so that when he wants to go somewhere, he has a crew to command.
All members of the group are Barons; they purchased the westernmost part of the Duchy of Urnst for a cool 3 million gold, and signed trade agreements with both the Duchy and with Greyhawk.
They also paid 1 million each to Greyhawk and to the Duchy of Urnst, as a "Sovereignty Recognition Fee," i.e., their barony is now an officially recognized political entity, with two allies and trading partners.
As part of the trade agreement, the new Barony has agreed to waive import taxes for both Greyhawk and Urnst for the first 25 years of its sovereignty.
The party always does their homework before setting off on dangerous quests; they drop an average of 10,000 gold per adventure on the use of their "Gather Information" skill checks in order to prepare themselves.
Oh, and then, they have to buy material components for spells, and for any magic items they might want to create, as well as paying for incidental expenses, such as bribing NPCs and what-not.
And then there's the cleric, who just bought a 500-volume reference collection of books about spellcraft and religion, so that he could get a +5 to spellcraft and religion checks made while at his temple. That set him back half a mil. (1,000 gp. per book.)
There's more that I'm missing, but like I said, I am working from memory here.
