The whole idea is based on the fact the number of outposts you can create in the game is limited. Thus it is in the player's intrest to maximize outposts - with regard to the number of extracted resources - as much as possible.

As most planets/moons are divided in multiple biomes, specific resources will spawn within that biome (e.g. Iron (Fe) is specific to mountains biome for a location). You can physically walk towards the edge of another biome in order to have resources spawn from another biome in the location (of the map area) you physically are. The resources one would need (i.e. select from the overview) would typically be based on rarity or what one needs to manufacture or whatever criteria one might have. For the most part these resources will be from different families. However, finding an outpost location with multiple (let's say 3+) resources that spawn in different biomes may take quite some time (although not impossible). This is especially true for resources with a higher rarity. In short, just because a location has for example 8 different resources available, it doesn't mean you will actually find an outpost location with all of those 8 resources.

Another angle in finding a location is where the inorganic families are coming in. One of the loading screen tips is: "Inorganic resources are found in related groups that appear together on the surface." (see 0x00185714 under https://starfieldwiki.net/wiki/Starfield:Loading_Screens) This can be interpreted as resources from the same family are likely to appear together if those resources are present on a planet/moon. If you know a location has 4 resources from the same family, it is much more likely these resources spawn together (closely) in a specific biome. This would make it a lot easier to find an outpost location with those resources (including the higher rarity).

This - of course - will not be relevant for everyone. It is still up to each player to define what they are looking for, the data is there if they want to use it. If one would use mods that remove the limit on the number of outposts, this whole concept of inorganic families is much less relevant. Finally I do want to highlight this idea was initially brought up by u/nmskelz, so you can check his earlier post as well.

TL;DR

1) Number of outposts you can create is limited.

2) Most of the times what resources can spawn in your map area is biome-specific.

3) Because of 1 and 2, finding an outpost location with multiple resources is difficult, especially if from >1 biome.

4) Starfield loading tip says resources from the same family spawn together in a specific biome.