Why do some companies invest so much on a TV series?

It *IS* difficult to believe that a company can benefit sufficiently from a high-cost ad on TV, but studies have shown that most do.

I see car ads constantly, but at 67, I've only bought 4 cars in my life, and I decided based on Consumer Reports.  

I see lots of LONG ads for prescription drugs for conditions that I don't have.

I wonder how often an ad gets me to buy something.  I DID just buy a flounder  sandwich at Popeye's.  The last time I was a Popeye's was 3 years ago.

The big money comes from syndication.  If you have a big hit it will generate income for decades after production ends.  Sienfeld  is a big example.  I Love Lucy is also one that has been making much more in syndication than it ever did during its network run.  It is profitable today and went off the air in 1957!   Even moderate shows sometimes develop a cult following and make fortunes after their cancellations.  Start Trek and Gilligan's Island are both examples of big profit syndicated shows that failed on network and were cancelled after three seasons. 

Anymore, most stations do not create their own shows, but outside companies do.  They sell the show to the network, who buys the rights to the show in their own country, or pay more and buy the rights to other countries too, or people in other countries can buy the rights to the show.

It is then up to the network to market the show, either through a subscription service, such as Netflix, in which you pay for it, through cable subscriptions, or free broadcast.  Cable and over the air broadcast will have commercials, which companies pay the network to broadcast.  The more popular the show, the more people will be watching it, and the more money a network can charge for a 30 second commercial.