Technique: Why it's important
Technique is skill that you develop and practice. For singing, technical skill gives you much more control, flexibility, and more choices for whatever you want to sing.
I like to think of practicing technique as Step Two during your warm up. Step One is getting your head in the right place for singing, leaving other chores and to-dos aside for now; warming up your body, stretching, and so on. Technique is skills-building practice to gain more agility and flexibility in your singing. It helps you with things like extending your range, working on your break areas (passaggios), belting, breath support, and a lot more. As Mark Baxter puts it in my book Your Singing Voice, "When the technique is there, the singer is free to focus on the real power of music: to create a moving experience, for both the musicians and the audience." (Gagné, Your Singing Voice: Contemporary Techniques, Expression and Spirit, p. 202)
When you do a quick warmup before singing and jump right into a song, we're skipping the skills-building part of your practice which is so important. Then when you're performing in front of an audience, your technique isn't ready and you won't feel as prepared or grounded to do your best singing.
Technical practice is about building muscle memory for your voice, for your whole body. Your body is your instrument. When you practice skills-building exercises that have nothing to do with a song, you're using all of the aspects of your voice and working them equally. This is how you maintain balance in your voice, giving you a flexible and well-prepared instrument.