Full Idea
Davidson's main argument in favour of his truth conditions theory of meaning is that compositionality is needed to account for our understanding of long, novel sentences, and a sentence's truth condition is its most obviously compositional feature.
Clarification
'Compositionality' is building up sentence meaning from components
Gist of Idea
Compositionality explains how long sentences work, and truth conditions are the main compositional feature
Source
report of Donald Davidson (Truth and Meaning [1967]) by William Lycan - Philosophy of Language Ch.9
Book Reference
Lycan,William G.: 'Philosophy of Language' [Routledge 2000], p.146
A Reaction
This seems to me exactly right. As we hear a new long sentence unfold, we piece together the meaning. At the end we may spot that the meaning is silly, or an unverifiable speculation, or not what the speaker intended - but it is too late! It means.