They make troop deployment harder
Decorations can make it harder for attackers to deploy their troops since they take a certain space on the map. Properly placed decorations can make it harder for an attacker to attack you or to win the attack.
They make the base look bigger
Decorations can make the base look bigger and so can help in reducing the number of attacks on your village. Some players might be reluctant to attack a base that looks big.
Some decorations intimidate attackers
Some decorations such as P.E.K.K.A statue can intimidate attackers. The fact that those decorations require a lot of resources to be acquired makes you seem like you are not an easy target.
They can store resources
You can sell your decorations anytime for 10% of their value. This means that if you get stuck, you can sell your decorations and get some Gold or Elixir back.
They make you seem stronger
Many players might assume that you bought decorations because you already bought everything else. This might intimidate some players and prevent them from attacking you. (See How to tell if Clan Castle is Full in Clash of Clans?)
They slow down troops
Decorations can slow down troops as they might have to move around their center. If you pace your decorations properly, you might slow down an army and defeat it easily.
They can distract attackers
Attackers will usually need a few seconds to asses whether they should attack your base or not. Decoration placement can provide a layer of distraction and can thus motivate a player to skip attacking your village. (See How to find and repair the gem mine in Clash of Clans?)