I already purchased self-employed for myself, and now I am doing my parent's taxed under a new account, but same email address as mine. why do I need to purchase 59 49?

Online accounts are separate.  And you pay for each one.  If you want to more than 1 return you should buy the Desktop CD/Download program.

Some advantages of the Desktop CD/Download program over the Online version,  https://ttlc.intuit.com/community/choosing-a-product/help/what-are-the-advantages-of-the-turbotax-cd-download-software-over-the-online-version/00/26486

Desktop you buy and install the program which can do unlimited returns.  The returns are only saved on your computer and can not be accessed from anywhere else.  And not saved at Turbo Tax.  So Be sure to save Frequently and make backups.  You get 5 federal efile for free.  In Deluxe and above you get 1 state PROGRAM download to prepare unlimited state returns (in that state).  Each state return including the first one is $20/25 to efile or you can print and mail the state for free.

Desktop also has many other features and advantages over the Online version.  You can do more than 1 return.  You can see and enter into the actual forms.  And you can use a lower version because all the Desktop versions have ALL the same forms.  You just get more help in the higher versions.  In online you have to upgrade to enter certain kinds of income.  Like Premier for investments.

You can buy the Desktop CD/Download program now here,

https://turbotax.intuit.com/personal-taxes/cd-download/