The purpose of DHCP is to share a limited number of IP Addresses between a number of computers. The number of computers accessing the network can exceed the number of IP Addresses avialable from the DHCP range.
As long as the number of active computers is not greater than the number of avialable addresses than there is no problem.
The short lease timeout allows the ISP to re-assign the IP addresses sooner rather than later.
If you turn off your computer the IP address becomes avilable at the end of the lease timeout.
Basically your ISP needs to be able to re-assign the lease as soon as possible. IT would imply they have a lerger number of customers on their avilable sub-net than most would have.
It should not be causing you any issues and as long as you are getting an address it should not be a problem.
If you are doing something that requires being able to access your computer remotely you should ask for (and pay for) a Static IP Address.