In Hindu tradition, the pooja room is an inevitable part of any house. Be it a big mansion or a small house, there is always space for the puja room within the house. Due to space constraints in the modern day, sometimes shelfs or racks replace the room but still there will be a separate place for the God in the house.
The puja room in the house was initially kept to symbolically represent the fact that the whole universe belonged to the God and we are infact just visitors for a few decades. In order to remind themselves of this fact, people had puja rooms in the houses. This was the place for all the worship and daily puja to the Gods.
With time, the concept changed a bit and God is treated as a guest in the house and we have become the owners of the house. Never the less the puja room is still significant. This room which is dedicated to the worship offers a calm environment which no other part of the house can match up to.
The reason why there should be a dedicated place for worship is that the human mind is a creature of habit. It responds well when some act is done repeatedly at the same place or at the same time. Hence a puja room provides that space to the mind where it can focus on the worship without any other distractions.
Also the need for a separate puja room comes from the fact that it should be kept unpolluted from the activities of other places like the bedroom and the bathroom in the house.
Also puja in hindu tradition being a daily ritual and an elaborate process during festivals, the puja room offers that space to keep all the puja materials and idols in a clean environment. Even some business premises and offices also have small puja rooms where the photos and idols are worshipped everyday.
You can also read about the importance of Puja Room, Why Hindus worship God only at Puja Room…?