I recently completed an entire read-through of the Bible. A lot of people do this over a year's time, but I've been slowly creeping my way through for several years now, just reading a little passage in the morning and sometimes in the evening. I think that this is a pretty vital thing for Christians to do. You could take your time like I did, but finishing the Bible in a year can help you retain all of that information. It's also a nice challenge for yourself.
I'd even say you should read through the Bible even if you aren't a Christian. It's always interesting to understand someone else's point of view, and there are definitely depictions of Christianity out there that are misleading and incorrect. It's always better to hear from the source. This list, however, will be targeted more towards reasons why Christians should read through the Bible.
Without further ado, let's begin the list:
1. Know what you believe.
This, I think, is the main reason why you should read through the whole Bible. If you're calling yourself a Christian, you should know what Christians believe (and why they believe it). You'll probably read some passages and stories that you may never have heard of before, or you heard them told a different way. Go through and look at the source of your religion yourself. Don't just piggyback off of what other people believe; make your faith your own.
2. Know how to respond to questions.
Have you ever met someone who wasn't a Christian? Most likely. You may have also met someone of a different denomination. People will have questions for you about your faith. They may even have challenges and misconceptions. If you don't know what the Bible actually says, how will you be able to respond? It's okay to admit that you don't know something, but being ignorant of something means you didn't take the time to learn it. And, of all the things to be ignorant about, you shouldn't be ignorant about your own faith.
3. Find answers.
This correlates with other reasons. For #1, you're learning about your own faith because, if you claim to be a Christian, you should know what being a Christian means. For #2, you're finding answers for other people. This is meant to be more personal. You're learning about your faith and finding answers for your own personal problems and concerns. You may be struggling with something, or you may have questions that you haven't been able to answer. If you've read through the Bible, you'll better understand what the Bible has to say about your concerns, and you'll be encouraged.
I encourage going through a program that helps you read the Bible in a year, or you could do it with a friend. Having structure and accountability (and someone to discuss your findings with) will help if the task feels kind of daunting. Just remember, the Bible is an amazing book that holds so much knowledge and beauty. There will be parts you don't want to read (either they're boring or heavy subject matters), but it'll be worth it.
Good luck!