Yes he is in every one in this world. He is present in every atom of this world and in every one’s heart.
So if Krishna is in every one then certainly he is in deity of temple also. And that deity is direct personal aspect of krishna which is very easy to understand and so we are able to connect with god. If you just see the all thing around us then you will perceive that thing not god .Suppose if you see tree ,you will think about tree,if you see bird ,you will think about bird and so on. So in all these cases you are not able to perceive god directly. Although if you learned the scriptures like bhagavat geeta then you can remember god by seeing all these thing but still you will perceive that material aspect also.
But what about when you see deity of god in temple. Immediately you think of god and connected with god. The devotee who has developed love for god , just by seeing deity they connected with god. And also in temple there are many person , who is there for same purpose to connect with god so you can associate with him and discuss with them and learn from them. This is not possible at all places. In temple i think there are more pure and spiritual atmosphere than any other places . That prasadam, that hyms, that bhajan, that arati and all other things. There are very positive energy so to connect and feel god , there are suitable place.
In short i can say , in temple all the spiritual energy is activated while on other places, it is deactivated or less degree of activation .
Thanks for reading!!