A necessary-use scenario is used in the pre-production phase of the software production.
In opposite to daily-use scenarios you will find out which feature of a program will not be needed frequently. A persona will use certain features of the program very often, others will not be used frequently.
For example, reading e-mail is one of the main features that the persona is using every day. Replying to e-mail and composing e-mail is comparably important to the user. In contrast to these functions, moving the e-mail into a specific folder is not used that frequently.
Nevertheless, this feature should be close by the main functions so it can be found and used when needed.
The users should be able to figure out how to use secondary features without external help from other users or manuals.