Good question. To start, Goodbudget uses your historical transaction data to run reports and to keep your Envelope and Account balances accurate. If you delete those old transactions, it will throw off those balances, and you won't be able to look at past reporting.
Also, deleting the transactions from your phone will delete them from your Household on the website as well.
If you'd still like to delete those transactions, you can do so by tapping on each one individually and then on Delete.