If I'm sending the customer to the offer page why should he see my pre lander?
In my case im collecting push subscribers but in another cases what would be the reason?
thanks.
- you can easier see which placements are full of bots (based on CTR for example)
- you can make funnel more consistent with ads and offer
- you can warm up audience before they see the offer for example by showing benefits
- you can collect retargeting database
- you can use backbuttons and other scripts which helps boost CR
I'm sure it's just the beginning of the list 
Some people understand this better when I use a real life example.
Imagine a car dealership with or without a sales person. Where are you more likely to buy a car?
In a dealership with a sales person, who will explain all the benefits to you, takes you for a test drive, tells you all you need to hear in order to buy the car and also plays with your psycho a bit by telling you just how great that specific car is and why you need to buy exactly that model?
Or in a dealership where the cars are shown, but nobody is trying to sell them to you and you just have to place the order with a bored employee behind the desk?
Most offers are designed pretty general, they look boring and do not really heat the user up ... so like a shop with no sales people.
Your LP is the sales guy, it has to heat the user up and make them believe this is an offer they cannot refuse 
Using a pre-landing page (pages) before sending the client to a selling site is a must. The pre-landers stir up the interest and persuades that the service is reliable. Also it explains how the service works.
Hey omerbsh, as matuloo and caravaggio pointed out, your pre-lander is used for warming up the customer to convert on an offer. The conversion rate would be significantly lower if you send customers directly to a sign-up page. Normally, there is no incentive or specific benefit for a customer to sign up on a page (would you sign up on a plane lead generation page?).
However, when you point out the benefits (they, out of all people, have been selected for a gift voucher) in your own pre-lander where you also create urgency (by using, for example, a timer), scarcity (saying there’s only 5 gift cards left) and place social proof (testimonials from people who have won), your CR will increase compared to if you direct link to the offer page. Furthermore, using a pre-lander can also influence the customer reaching certain KPI’s, such as re-bills for Nutra. For example, if you state in the pre-lander that 90% of the women who bought a Diet pill experience the effects after 30 days, that will prevent people cancelling the product if it doesn’t show results in 1-2 weeks (thereby making sure that they will be re-billed)
Not to get overly dark, but this is a POP/PPV section so let's just call it what it actually is.
Pre-landers on POP are predominantly used to deceive a user into thinking they've gained some sort of a prize by answering a couple of questions, spinning the fake wheel, or magically selecting the right prize box. Otherwise it would be difficult to get them to fill out and submit a form without any sort of reward promise.