If your teeth are very crowded or proclined outwards then teeth removal is important for achieving good results, pushing teeth back and gaining proper lip competency/seal. In such case, If he does not do the extractions then the doctor won't get enough space to align your teeth nicely.
Next Steps
Informative animated videos are available on youtube. You will understand better.
Health Tips
Make sure you get your treatment done from an Orthodontist only.