What is the need of having a key schedule before executing AES Can someone explain to me why does using a key schedule make's AES more secure if instead of calculating and adding your round key's, you just keep on adding the cipher key. ?
Not using separate keys for each AES round would make your cipher vulnerable to slide attacks.
Using two plaintexts M and M' with M' being the result of an AES round after calculating M as input, you can differentiate between those plaintexts by calculating the output of an AES round which takes the ciphertext C as input.

Due to the birthday problem, this attack schould reduce the complexity of breaking the AES to $\mathcal{O}(2^{n/2})$.

Source: Silde attack