Daily websites are hacked, and dumps of the email/password files are placed publicly online. For this reason, it is not a good idea to use the same password on all the sites you access, otherwise it would be easy for someone to use one of these passwords to attempt to access another account of yours, for example your bank, or social media account.
The best plan is to have a different password per website, but that is a lot of password to keep track of- unless you use a password manager. 1password makes the process so seamless that you never actually see a password, your browser will fill in the passwords for you, and keep them secure (you need to enter the master password once per session). It can also transfer your passwords across devices, so you can access them on your phone, or your pc or mac. Splash ID does a similar job, and there are plenty of other password managers – its just a matter of finding the best for you, but these two should be top of your list