Why do people expect coherence in the bible? Lots of questions on this site are asking how some text from the old testament fits to a contradictory one from the new testament or a passage of one book to the other.
Why do people expect to find this kind of coherence in the bible? Clearly the different text passages are written several hundred years apart, by different people with different interpretations of the exisiting scriptures in mind and with a varying set of personal beliefs and values.
Different denominations and groups within denominations hold to varying doctrines of inspiration and inerrancy. If that denomination believes that God inspired the writer and that some version of what that writer wrote down (the &quot;autographs&quot;) is inerrant, then they must account for apparent discrepancies. The logic is:

God only speaks the truth
God is logically consistent
The Bible contains all and only the words God intended it to have
Those words must be true and logically consistent

Some people believe that God dictated the Bible, word for word. That is not the prevailing view. It is a view that some sects and some people in other religions believe about their Holy Texts.
One model that many Christians accept is called &quot;Verbal Plenary Inspiration&quot;.
This model means that every word is inspired, all the words are useful (Old and New Testaments) and authoritative and the Holy Spirit was involved. It allows for the personality of the speaker (say a prophet) to shape the vocabulary choice, organization and style of the message while simultaneously capturing absolute truth precisely.
Other people allow for errors in science and history in the Bible and say only the spiritual truths are inspired, the rest is the fallible opinions of the writers.
Depending upon your view of how the Bible came about and what measures God has taken to safeguard the words from corruption, the consistency of the different parts of the Bible may be a small or large issue for you.
Scripture itself makes claims about which views of innerancy and inspiration and transmission it endorses:

&quot;Heaven and earth will pass away, but my words will never pass away.&quot;
(Matthew 24:35)


16 All Scripture is God-breathed and is useful for teaching, rebuking,
correcting and training in righteousness, 17 so that the servant of
God may be thoroughly equipped for every good work. (2 Timothy
3:15-17)


13 And we also thank God continually because, when you received the
word of God, which you heard from us, you accepted it not as a human
word, but as it actually is, the word of God, which is indeed at work
in you who believe. (1 Thessalonians 2:13)


21 For prophecy never had its origin in the human will, but prophets,
though human, spoke from God as they were carried along by the Holy
Spirit. (2 Peter 1:21)

If you take the above Bible quotes at face value, it narrows down the acceptable range of views that an orthodox Christian may believe considerably. For all that reduced set of possible ideas of inspiration, the presence of actual inconsistencies in the Bible is impossible, hence must be addressed by attempts to explain the Bible passages in such a way as to remove the apparent inconsistency.