1, why is meiosis necessary 2, how are mitosis and meiosis different?

Answer 1

Meiosis is necessary so an organism can form haploid gametes that can combine with the gametes from another organism of the same species to form a new, unique, individual (sexual reproduction).
Mitosis is the exact replication of a normal, diploid cell. Thus, meiosis is often called "reduction division" as opposed to normal replication and cell division, which is mitosis.

Answer 2

(1) Meiosis is necessary for sexual reproduction to work. The gametes (sperm and egg) have to have half the DNA of a normal cell, or else the amount of DNA in the cell would double with each generation and the genome would very quickly get unmanageably huge.

(2) In Meiosis you end up with cells that have 1/2 the genetic material of the original. In mitosis you end up with cells that have all the genetic material of the original.