Why was Eleanor Rigby a turning point fo The Beatles?

Paul McCartney was generally open to expanding his musical horizons. Even McCartney, however, resisted George Martin's suggestion that the use of strings would enhance certain compositions. "No vibrato, George!" he insisted. "I don't want to sound like Mantovani."

It was the arrangement of YESTERDAY which convinced McCartney that strings could work in pop music. This allowed Martin to experiment further, drawing more on contemporary classical music. For ELEANOR RIGBY Martin wrote a Bernard Hermann-influenced string part to bring out the bleak, unsettling themes of the lyrics (death and loneliness). 

Significantly, The Beatles only contributed vocals to the recording of ELEANOR RIGBY. This was the first record on which none of them played their instruments. The gap between what they produced in the studio and what they could recapture as a live band was widening.

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