This page will give you an understanding of the uses and some strategies of foundries.
For further information on asteroid fields, please check the Dark Forest Wiki.
The main function of foundries is finding upon artifacts from them - they are the only celestial body within the game where artifacts are found.
To prospect (find) and artifact on a foundry, you must have your ‘Gear’ spaceship located on a foundry you own to prospect an artifact (no, you can’t steal other peoples artifacts just because your Gear is on their foundry). The artifact found upon a foundry is randomized (unless you are playing in dfdao’s Arena with deterministic artifacts turned on - where the same map will spawn the same artifacts every time its recreated/played). When the foundry is prospected, the rarity of the artifact found corresponds to the foundries level (0-1 Common, 2-3 Rare, 4-5 Epic, 6-7 Legendary, 8-9 Mythic). The biome-type of the artifact is determined by the biome of the foundry - i.e. Ice artifacts are found only on Ice-Biome foundries, however they can be used on any biome of celestial body.
Foundries have a multiplier of 1x for all of the default stats, giving them the same stats as planets. However, the trade-off for finding artifacts upon them is that they cannot be upgraded.
Due to the randomized aspect of foundries, it is often not optimal to focus on them in the very early game, your energy is generally better spent aiming for other celestial bodies. Foundries which have already been prospected are rarely of any value since the only other resource produced is energy.
However, once a small-medium sized empire has been set-up, it can be extremely valuable to begin prospecting many low level foundries for the chance to find specific artifacts - Wormholes and Photoid Cannons are all extremely valuable, regardless of their rarity, so even finding common ones nearby from lvl1 foundries can give you an edge over your adversaries.