They're not crucial. Are you sure you did any research?

It is crucial to have a commitment in the block header which represents all the transactions in a way that allows proof that the transactions have not been modified after the block was created. A hash (SHA2-256 is used) serves as a proof because it is not forgeable 

It is crucial to minimize the size of the block header, so that block-level chain proofs can be very fast. A hash is only 32 bytes. It serves as an efficient compression of several thousand transactions 

The hash commitment which represents the transactions could be calculated by serializing all the transactions in order and making a single hash calculation
The hash commitment which represents the transactions could be calculated by concatenating all the transactions' ID hashes in order and making a single hash calculation

Merkle trees have benefits for coders - a clearer separation between the function of a transaction and the hashing functions used to implement immutability in the chain. This benefit persists over the long term, reducing the possibility of bugs being created by future software maintenance

Not crucial, but very useful