Are job fairs truly organized for recruitment or just an opportunity for advertisement and promotion of companies instead of using other strategies?

Somewhat true. Most of the organization will attend the fair for customer or people visibility rather than actual recruitment.

Some companies may be use for advertising but lot of companies use for recruitment purpose. 

Actually,  high level management wants to hire winning horse and provide equal opportunity for all but low level supervision try to hire their relatives,  friends and costs. 

This is not fair but fact is fact. 

I DIDN'T JOIN ANY OF JOB FAIRS BEFORE BUT I THINK IT IS JUST OPPORTUNITY FOR ADVERTISEMENT

For both reasons if I may say,

Businesses are established to make profit and get returns for its investors and share holders so every necessary marketability and promotion medium to bring more sales and expansion is been deployed.

Though some organizations have taken it to the extreme the way they do it, but we know all profit making ventures need money to pay its rents, pay salaries/wages and manage the organization. So in this regard, they most times use all available medium to promote their business which does not take out the 'job fairs' from it and sometimes CSR activities.