Filing a flight plan for a cross-country flight is important for a number of reasons, VFR or IFR. For one, if there's a mishap anywhere, someone's going to begin looking for you immediately if your flight plan is not closed. (That's why it's so important to remember to close them!) Also, it's recorded evidence that you followed the FARs: "to obtain all relevant information about the flight." That's your responsibility as PIC. Maybe you did check the weather and decided it was OK
to fly, but if you have an incident and weather is part of the cause, can you prove you checked the weather? If it's indicated on your flight plan, you can.
That's true, but be careful how you do it. If you call a Flight Service Station and get a weather briefing from a specialist, there's a recording made with your voice, and either your name or tail number, indicating you really did check the weather. If you log on to DUATS online, the same thing happens. But, if you just check the TAFs and METARs, and decide it looks OK
, that's not going to be recorded by the FAA. If something happens, it's always better to have covered yoursef.
To fly IFR, you HAVE to file a flight plan, because you will be controlled by an air traffic controller for the entire duration of the flight. For VFR, it's not absolutely required (unless you're in the ADIZ) (or you're a student pilot, because your instructor is going to make you
Also, there are pilot and aircraft certification differences for VFR and IFR flight. All these things are well-documented in the FAR
/AIM. If you ever want to get into non-simulated flying, it might be worth the investment. Plus, you can buy these at any flight school, so it's an opportunity to head to a local FBO and maybe see some planes up close.