Why have the courts adopted the principle of proportionality for reviewing acts of public authorities that interfere with human rights?

This question is from the Public Law, Rights and Remedies Module. In answering this question, please refer to case-law, the judges’ reasoning and any underlying considerations that may exist.
Proportionality pre-HRA period. Criminal Justice Act 1991: sentences should be commensurate with the seriousness of the offence e.g. R v Barnsley MBC ex part Hook [1976] and R v Admiralty Board of the Defence Council ex part Coupland [1996]
How proportionality has overlapped to some extent with Irrationality/Wednesbury Unreasonableness
Difference between proportionality and Irrationality/Wednesbury unreasonableness.
Proportionality in the post-HRA period. Justifying proportionality : the ex part Daly case; the structured test of proportionality examination; issues of discretion or due deference i.e. the limits of proportionality and proportionality in practice.

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