Why it is necessary to have elections at regular interval?

When elected officials know that they are going to face another election, that is coming up in the not too distant future (let us say, within 5 years) then they know that they are accountable to the voters. If the voters are not happy with their job performance, the voters may vote for someone else. This means that the elected officials will feel some need to do what voters want them to do. If, however, government officials do not have to face re-election and just hold on to their jobs forever, or for some very long period of time, then they may feel no obligation to do what voters want them to do, and may instead serve their own personal purposes. Those purposes usually turn out to be the vast enrichment of themselves and their families and cronies, at the expense of the public.