I always wonder that what UK immigration at airport check in their computer and why they type our answers in their PC?
For the first part of your question "what UK Immigration...check in their computer"...
Immigration Officers at the border are obliged to examine all passports at ports of entry and to check them against the records stored upon our database; this is accomplished by means of the scanning process you have observed. I can confirm, however, that the system employed for the scanning procedure has a read-only capability and does not record any details from documents which are scanned.
It is necessary at times to hold details of individuals in order to maintain the integrity of our borders and the general security of the United Kingdom. Such information will come from different sources and is regularly monitored, updated and removed from database records when deemed appropriate. I hope you will understand that, for reasons both of confidentiality and security, I am not at liberty to discuss the nature of any information held on our database, nor discuss the sources from which such information originates.
The scan at the Immigration Officer's station is very rudimentary. They check their database to see if there is any matches for your passport or biometrics. If there is a match, the computer will return a stop/detain indicator. The passport scan also returns all of the basic information from your passport to help them detect forgery. If there is a stop/detain or something else where they need to check further, they will take the passport to a secured station where more detailed information is returned. That's why they sometimes disappear for a moment.
Also, if you have an entry clearance (visa) in your passport, the system will verify that it was properly issued and has not been cancelled. They may also want to pull up the full Proviso record and if so they will take it to a secured station for that reason.
For the second part of your question: "why are they entering your responses in to the computer". They do this in order to have a record of the interview. They may also enter the number on your landing card (the white card you filled out before landing) in case they need to find it again.