what's the difference between dim as and dim as new?

There is no difference with value types (Integer, Double, Structures, ...).
The instance is created anyway in both cases.

There is a difference with reference types: Without new, only the space for
the variable is reserved (currently 4 bytes = 32 bits). No object is
created. The variable contains Nothing (= no reference). With New, it is
equal to
dim o as type
o = new type
which means it also creates an object and stores the reference to the object