Just the same way the grant of copyright is justified as a form of reward, incentive, and a natural right of those who create original works. The public has the right for a number of different copyright exceptions which allow a person to copy protected works without acquiring the owner of the copyright work without infringing copyright.
The scope and extent of these exceptions vary. Bently and Sherman state that this is a reflection of the variety of purposes which these exceptions serve.
Some exceptions to copyright are meant to promote and encourage the creation of new original works, especially where copying is not identical and is transformative.
Other exceptions are established to solve possible market failures that could arise in situations where the use is too nominal leading the cost of any transaction to establish an agreement between the copyright owner and the person wishing to use the work uneconomical. This can be seen in the exceptions related to subtitling (s74 CDPA) and recording of broadcast by educational institutions (s35 CDPA).
Copyright exceptions are also needed to protect other non-copyright public interests such as privacy (e.g. private study (s29)) and freedom of expression (e.g. criticism and review).
They are also used to prevent monopolies of intellectual works from being abused. (e.g. decompilation of computer programs s50B and retransmission of cable programs s73).
These exceptions are necessary for preserving material of cultural value (e.g. exceptions relating to folksongs s61 and archiving exceptions s75).
Finally, some exceptions encourages collecting licenses. (e.g. compulsory licensing of lending of works s66).