Why do we play out of a range?

Is it because it bolsters our original holding?

A “range” is just a set of hands. It need not be a continuous looking set on a hand chart. In most spots the proper range will be, but if you folded everything preflop except AA and 72o (as a bluff), your opening range would be AA, 72o. You could throw in other “disconnected” hands and have a range like AA,JJ, QJs, 72o, and it would still be a range.

That said, in general there are terms for ranges that are more “connected” and ones that are more “disconnected”. Roughly speaking a connected range is a linear or merged range. It contains top hands plus mid-range ones. A less connected range (but I’m not sure spots with completely isolated ranges actually exist) contains very strong hands plus complete air - that is a polarized range.

This is a gross oversimplification but preflop we tend to want to use more merged ranges. Equities run fairly close so we don’t want to only play very strong hands plus a few weak bluffs. We play more middling type hands. Again oversimplifying, we tend to be more polarized on the river. We want to bet hands that will either get called by worse (our very strong hands) or that will cause better hands to fold (our bluffs). We tend to want to check our mid-strength hands.

To put it simple, we need to know what hands and how much of them we have in order to understand how we are going to play the hand we currently have. It is mostly a balancing thing, if we don't know how much value we have, we are going to overbluff / underbluff and won't use correct sizings (either won't be extracting value where we technically can get it or we'll always go purely for value with big hands we will be just telegraphing our hand strength, while not supporting our large-bet hands with enough bluffs), all of which is going to be very unoptimal and can lead to you being exploitable.