What are the advantages of an SSD and is it really necessary? I shall use it to install my OS...I know that it can help boot-up and shut down PC faster...but aside from that what else

Now, if it is good to have one, please help me choose between the following:

1. 120GB Kingston for 4,250.00
2. Sandisk 128GB for 4,400.00
3. Crucial 120GB for 3,900.00
4. Corsair 120GB for 4,800.00
5. A-DATA 120GB for 3,900.00

I'm leaning towards Crucial and A-Data since both are the cheapest among them....TIA