I would guess that this is a question that strikes most Christians at one point or another.
And in practice, most Christians don't really take much notice of the Old Testament. The old Church of England was an exception - because a regular church-goer would hear much of the OT read-through each year. However, probably all Christian denominations focus on the New Testament, and some almost exclusively so (for instance, some modern evangelicals).
Yet when we pick-up the Bible - most of it is Old Testament. Why so much?
The usual explanation is the the Old points-to the New... OK, but why is there so much pointing? Surely a bit of pointing would be enough - not hundreds of pages?
Well, there it is; and it is apparently up to us to discover what is the best use to make of the OT.
My suggestion is to regard it as the history of God's interactions with His people; told mostly from the perspective of those people - in other words from an (inspired) human's-eye-view and not, therefore, from a God's-eye-view.
(This is a possible to answer to why the OT is so long. So we can 'correct-for' the multiplicity of different perspectives.)
If regarded in this fashion, the Old Testament looks like a collection of examples of the constancy of God's personal loving concern for His people - this a constant factor lying behind what are depicted as great variations in His people's understanding, love and concern with God.
Since there are multiple examples over time, and multiple forms (annal, fable, poem, prophecy etc) there is redundancy in the OT; but since there is redundancy (as well as multiplicity of forms), we are not (I think) supposed to regard the OT as either wholly essential, or complete; nor as saying the same thing as the NT; nor that all parts of the OT are saying exactly the same thing (rather, they illustrate broadly the same general theme).
I think we can (and perhaps should) reasonably regard the OT as a resource or compendium to be probed and explored for particular and personal helps in our understanding of God's relation with His people (and therefore ourselves).
In practice, this is probably almost exactly what is done by most Christians - but I don't think we should feel so guilty about it - I mean about picking and choosing within the OT, and leaving-out a lot of it altogether!
In a sense, that is probably what the OT is actually for! Something for everybody, but everything for nobody.