Do I really need a USB scanner?

Depends... so let's go over all the variables and you can decide if you really need one.

Reasons for getting at least one laptop connected barcode scanner
  1. A good scanner can be faster than scanning with a phone. Assuming the laptop has a good wifi connection, expect that it will be 20% faster than the phone.
  2. It makes accepting the student permission form a breeze. The first scan brings up the students account, click a check box, then assigned the barcode tag number by scanning it. Done! Now imagine typing the number on the keyboard, but there is a line of 15 students and 5 minutes until the bell rings.
  3. The laptop can update student accounts that the mobile app can not. Need to locate a student account and give them a new barcode number, update a parents email or phone number? This type of access requires a laptop and the scanner makes locating the student record that much faster.
  4. No typos. Assuming you are hand entering the barcode number on the computer you might accidentally input the wrong number. Since the barcodes are sequential this might cause a notification to go to the wrong parent.
  5. The cellular network at the school site has a very weak signal or no access. Be sure to check both major carriers ATT and Verizon. It's very possible one will have an amazing signal and the other might not even work. As always go to the exact location you want to scan at to do your pre-testing there.
Reasons you might NOT need a laptop barcode scanner
  1. Limited budget or no laptop to connect it to.
  2. 200 or fewer students (per scanner) are expected to scan in daily.
  3. You don't mind doing the administrative things (new accounts, change emails, etc) at home.
  4. You have great cellular network coverage at the school and a number of volunteers with smart phones ready to go.
  5. Your school has more than one main entrance.
Side note

I've noticed more schools starting a pilot scanning program with just the mobile scanning apps and an iPad. The iPad give you web access to the accounts and the mobile apps allow for easy and free scanning. Those scanning with the mobile apps will get faster once they have figured out the correct distance and light required. Newer faster phones with better cameras are the best for quickly capturing and processing the barcode.

You can always decide you want a usb connected scanner at a later time.