Vocal warm-ups are an important part of singing, but they offer many other benefits in a choral setting. Choral warm-ups are an important and powerful tool to get your group singing together with a good tone, resonance, and proper breath support (source). Plus, there are many ways to include pedagogy and teaching in these first few minutes of the rehearsal that will save you time later.
Warm-ups are often an after-thought in rehearsal planning and many choirs tend to do the same batch of exercises every week. There's nothing wrong with this, per se, but if you spend just a few minutes thoughtfully planning warm-ups that prepare concepts from the repertoire, warm-ups become a valuable teaching tool. There are several benefits to this approach:
Your choir will be more engaged. By mixing up the warm-up exercises each week, you offer your choir a new challenge every time they come to rehearsal. They will likely pay more attention and be more engaged while singing.
You will spend less time introducing new pieces. By preparing new concepts in the warm-ups (e.g. triple meter or vowel placement or a melodic phrase), your choir will be practicing a challenging spot from a new piece without even realizing it!