The hechal (central room) of the bes hamikdash had its main entrance on its eastern end. To either side of that entrance were cubicles, and the main-floor cubicles on each side had a door to the east also. Thus, the eastern exterior wall of the hechal had three entranceways visible. The northernmost of these, which entered into a cubicle, was unlocked and opened every morning before the korban tamid (morning sacrificial offering) could be brought. Mishnayos Tamid chapter 3 mentions that fact, and mentions, also, that door's means of opening. While commentators differ on what is meant there — the Rav says (ibid.) that one had to reach through a hole in the wall to unlock the door from the inside, while the Rashash says that one merely had to bend over to use a keyhole near the floor — it was by all accounts an awkward way to unlock a door. I haven't seen anyone, however, explain why that door had such an awkward means of unlocking from the outside, especially in light of the fact that the door was unlocked daily from that side. Why did it?
The Rav (see Rav mishna 2 also) explains that there were two locks on this door,there was a lock on the inside toward the bottom of the door and one on the outside, facing the person entering. The inside lock would be opened by sticking your arm through the hole and reaching downward to insert the key and the outside lock was unlocked with another key easily.
Why two locks, one on each side? Why specifically was there a lock also on the inside as was asked?
I would guess that the idea is to make it as difficult as possible for someone to pick the locks and open the door without the actual key. Because even if you could pick the lock on the inside, which would be difficult on it's own and this would be one measure of security, you would have an even harder time to hold the pins of the inner lock in place while picking the lock on the outside.
The Rambam (Mishna 2) also explains that there were two keyholes, presumably on the outside, one at the natural height so a person can just reach out to insert the key and the second near the floor so that it should not be easy to unlock. This too can be explained as a method to make it quite difficult to pick the locks.
I am understanding locks in those days did not have a key that you can turn to unlock and then remove the key with it remaining unlocked, rather the key would be inserted pushing pins allowing you to move the bolt that is locking the door.
Similar to this link: https://www.smith.edu/hsc/museum/ancient_inventions/hsc09b.htm
I found a similar idea in this book: Google Books-Life in biblical Israel (pages 31 and 32)