Here is a few reasons you want to send dispute letters instead off disputing online. 

PAPER TRAIL- being able to prove you have attempted to dispute items on your credit report for cases where the bureaus do not respond or we have to threaten to litigate. 

Explanation- disputing online forfeits your rights to elaborate when disputing your accounts. Online makes it easier for the credit bureaus to deny your dispute request.  

Laws- the FCRA (Fair Credit Reporting Act) does not include the same protection when disputing online as the do with mailed in disputes. Even if an item is deleted from disputing online, it can be temporary deleted and come back on your report later.