It forces you to explicitly declare all variables.
What is the difference between explicitly declaring and implicitly declaring a variable?
Explicitly declaring a variable:
Dim anInteger As Integer = 1234
Implicitly declaring a variable:
'Did not declare aNumber using Dim aNumber = 1234
Therefore, you should always have
Option Explicit On as you could misspel a variable during assignment, which cause your program to behave unexpectedly.