While processing a column or many columns or a list for processing into any loop or apply group of functions, why is it necessary to convert it into vector? what's the rationale behind it.
You wouldn't necessarily. For example; columns typically are naturally vectors for example, and converting them to vector most often will give an identical result; a wasted operation.
(mydf <- data.frame(a=1:5))
identical(as.vector(mydf$a),
mydf$a)
Based on that its hard to know how to answer you.