You have to have a job offer in order to apply for a work to residence visa, or any other work visa. You can apply for Residence without having a job offer.
If someone gets a skilled job offer, in order not to keep the employer waiting longer than necessary, most people apply for some kind of work visa as well as Residence. The work visa can be processed fairly quickly, so the applant can have started work while the Residence application, which takes longer because of deeper checks, ticks away in the background.
A Residence application can include the main applicant's partner, and any dependent children (see the INZ dependent child definition). A work visa applicant can sponsor their partner for an open work visa (so s/he can take any job), school-age children of a work visa applicant can get student visas, and pre-schoolers go in on visitor's visas.