push email will be less strain on your batter and the network than, say, the default email that has to create a connection for each interval.
basically, the phone just stays "logged in" to the Google Cloud. your phone only updates as information arrives.
for the default email client, it has to establish a connection, log on to the server, check for new messages in each assigned folder, download the messages and send any messages waiting to be sent.
if you have multiple accounts in your email client, it does this for each account for every interval you have set.
think about how many data connections you'd have to establish and wait for if you had 20 accounts syncing every 5 minutes (extreme situation, but you get the point).
that's why i advise everyone to stop using email the old way and allow gmail to check your POP3 accounts for you.
