You wouldn’t, you would build an Arsenal for the upgrades it provides. This just makes it so it’s not completely useless once all the upgrades are researched.
There are also a few factions such as Japanese or Ethiopians that can build Arsenals but not Artillery Foundries so this would be a way to give them access to Petards.
all the other arsenal upgrades provide a more solid and simple buff to a single unit type’s stats or modifiers. This one would feel a bit out of place with opening up your tech tree to train units from buildings you previously couldn’t. I feel like this would be better if the effect was stronger and it was a HC card instead. It could be a card for all euro civs that in addition to the falconet thing would buff petards to make them a more viable option. Kind of like what grenade launchers does for the grenadier.