What are conventions for??

The main work of the convention is for delegates to formally nominate a presidential candidate and approve a party platform, which party platform committee members voted in support of earlier this month.
In some rare cases, no political candidate has a majority of delegates entering the convention. That’s what’s called a contested convention, and delegates select their party’s nominee through one or more rounds of voting. If a nominee withdraws, there would be a process known as an open convention, where a new nominee could be chosen on the fly, historically with behind-the-scenes negotiations and bargaining.
In this case, when a general-election candidate such as Trump has been determined as a presumptive nominee before a convention, the pageantry involved in the event may seem unnecessary.
But politicos argue that while the actual work of a convention could be condensed, the multiday agenda provides major opportunities for lesser-known party members to introduce themselves to American voters and allows the party to energize their base and lay out its vision for the country.