Photos can represent memories that are difficult or impossible to replace, so having another copy is a sensible precaution.
Why make a backup?
Phones can be lost, damaged or replaced. A backup gives you another copy if something happens to the device.
Cloud backup
Many phones can automatically upload photos to a cloud service. Check that synchronization is enabled and understand how much storage is available.
Cloud synchronization is convenient, but it is worth learning how deletion and synchronization work before assuming that every deleted file will remain safely stored forever.
Computer backup
You can also copy important photos to a computer. Organize them into folders by year, event or another system that makes sense to you.
External storage
An external drive can provide another copy. Keep important backups separate from the phone so that losing the phone does not also mean losing every copy.
Check your backup
A backup is useful only if the files can actually be recovered. Occasionally open a few backed-up photos or documents to confirm that the copies are accessible.
Use more than one copy for irreplaceable files
For especially important memories, having copies in different locations provides more protection than keeping everything on one device.
The best backup system is one you can understand, maintain and verify regularly.