Just consider the real life example:
Suppose you are running a Store in your city and you are using the Two Tier Client Server Architecture. After some days, due to hike in market you decided to change the prices of some products. Your application and Business Logic is installed on the machine, so u have updated the price on your machine which is nothing but client machine.
Now suppose you have opened a new branch of the same store in another city which is again depending on Client Server Architecture. If again you need to change the prices then you have to visit both the store since the application and business logic is installed on the respective client machines. Just imagine if u have number of stores across the country then it will be very hectic for you to maintain the uniformity in the prices, since you need to update every machine.
Hence the Three Tier Architecture came into picture.
The Application or business logic will be installed only on Web server. The client at different location will be accessing by sending the request to it. Now whenever you want to change the prices in your store you just need to change it at the Web Server. The updated price will be served to all the clients. So it has reduced a lot of efforts.
