It's a strategy of competition?

For example, i choose shampoo, nowadays there are so many brand of shampoo, drop his price (shampoo price), or sometime the just add the content of shampoo or maybe they give a gift or sometime they sell a packet ( buy 2 get 3).
I just think how they can get a profit from that way?
Hmm maybe because of the decrease price of things to make shampoo, if not, how?
Hmm second opinion, maybe they just get small profit and the important thing is they can sell as much as possible. So from that selling product they can get profit as much as possible.
The funny thing that i can see is, about the shape of bottle, you can see it's almost same between one product shampoo with the other product shampoo. And if one brand launch new bottle, the other brand will launch new bottle also. Is it because of that brands is in the one company or they book bottle from the same bottle's company?