It is very risky to host a server on your pc , As either u have to expose your IP address to WWW or open some ports which can let hackers hack into your pc. Also you have to regularly patch and install security updates,you need to keep your pc on for 24x7 which will again cost u electricity bill, even if u can afford to keep ur pc ON for 24x7, there will surely be atleast some powercuts.
Say , for example u are on a tour and u want to access ur music collection. how can u be sure that ur home pc is on and connected to net? The best is to pay for some cheap hosting. Hosting space are quite cheap nowadays and offer great security,peace of mind and most importantly many provide a 99.9% Uptime.