If you don't want to just play muzak but also wish to make announcements, a mic is crucial. Mics for 100V PA systems come with a panel of push buttons that allow you to select individual zones for your announcements, or to broadcast your announcements throughout the sound system.
The option to connect one or several handheld mics may also be of advantage to you, either at the mixer or the PA centre. If these are radio mics (e.g. Sennheiser Powerpack, see below), you can even launch your announcements from anywhere within the area of reception (useful e.g. for announcers in the middle of a football pitch).
In some setups, you can prioritize the mic, i.e. whenever the mic starts transmitting, the music fades out and resumes once the announcement has been completed.
You mustn't, of course, forget the cables from the amp to the speakers and/or groups. As we have mentioned several times, you can do without large-diameter cables (0.8mm as opposed to 2.5mm and more), which reduces your costs one the one hand and increases the maximum useful cable length on the other.