@VKS,

Hm... It's just used for incrementing the output column to avoid manual input and keeping a single formula will be easier one in case of any changes instead of going through each cells for changes.

For example in this OP's case the expected output column results are 4 and 5.

So the formula should be

In A3 cell
=VLOOKUP($B$3,DATA!$A$4:$F$13,4,1)

In A4 cell
=VLOOKUP($B$3,DATA!$A$4:$F$13,5,1)

In the above case it requires two formula's for getting two different column's output. What happens if we have more formula like this... It requires more time to check each formula whether the formula's are unique or any manual change has been made.

So instead of using multiple formula's based on the requirement, making the formula single will help us in case of any change in future.

Row(A1) will get the result as 1
Row(A2) will get the result as 2

In the case case
Column(A1) will get the result as 1
Column(B1) will get the result as 2

So here the resulting columns are just a incremental series so just added row(a1) and 3 is a constant value for getting the result as 4. When it applied to next cell the row(a1) will become as row(a2) and the result will be 2 and addition of constant 3 will result as 5.

Hope I made it clear