Tyres Question - Change one or two

[Kisu_san]

My car have

285/45 19 V 107 Michelin Diamiris summer tyres at rear. Thread is half worn.

One of the tyre got punctured and it is beyond repair. (at least this is what mechanic told)

Question:

1. Is it advisable to change only one tyre?
2. If above Q1 is yes, then same brand as other or any brand?


Mechanic told me to change both. I did not understand rationale. How does this make a difference or impact car/