To travel to many countries including the UK, US, Australia and Canada you must first apply for a visa to be permitted to enter the country you are travelling to.
The visa must be approved by the sovereign nation and the outcome will be based on your individual circumstances. A visa is essentially permission from a foreign country to enter their nation.
Whilst passports allow you to travel its purpose is generally proof of identification however a visa is a separate document. The visa will hold your reason for travel and must be produced upon entry to certain countries. Each country that states that you must have a visa for entry have their own rules and you will need a separate visa for each country that you travel to. For example, you would not be able to use a UK tourist visa to enter the US and vice versa.
A visa will indicate to the border control what the purpose of your travel is and based on the type of visa that you have you will only be permitted to partake in certain activities.
If you have a tourist visa to the US you will be prohibited from working, studying and setting up any form of permanent residence within the US. Any individual who does not adhere to the terms of their visa will be deported back to their country of origin and it will be held on record.
Different types of visas are valid for varying periods of time and you must not overstay in any country, if your visa allows you to stay in a country for 90 days you must not attempt to stay past this time frame.
Visas allow the sovereign state to monitor who is in their country and what they are there for and how long they will potentially be there. Without visas, there would not be any visibility for governments and there is the potential for dangerous individuals to enter countries and commit serious crimes.
In conclusion a visa is a monitoring tool and is there to protect countries and individuals from harm