when is it important to use different hosting accounts?

Is it important that I have my sites setup on different hosts if I am going to have them all share the same tier 1s? would google be able to put two and two together? I have noticed a few members talking about spreading their sites over different hosting accounts, but I am not sure why people would need to do this. if this is not the case, could you give me some examples of when this would be important? just making sure all my work isnt going to go sour because of a stupid mistake like that.

is good to have different hosting accounts so you could link the sites to each other .
Another good reason could be if your target different countries with each site to host it in that country