Using WHERE / AND in Joins
As Lennart has suggested, use a minimal example and see what happens.

create table accounts(account_id int primary key, sales_rep_id int);

create table orders(order_id int primary key, account_id int);

insert into accounts values
(1, 321500),
(2, null),
(3, 2000),
(4, 3000);

insert into orders values
(1, 1),
(2, 1),
(3, 2),
(4, 2),
(5, 2),
(6, 3);


There is a LEFT JOIN involved, that means it returns always all rows of orders table, plus those rows of accounts table with the same account_id. But, the WHERE clause filters final result to those with sales_rep_id = 321500. 

order_id | account_id | account_id | sales_rep_id
-------: | ---------: | ---------: | -----------:
       1 |          1 |          1 |       321500
       2 |          1 |          1 |       321500


However the second query returns all rows of orders table, but only accounts with the same account_id and sales_rep_id = 321500.

order_id | account_id | account_id | sales_rep_id
-------: | ---------: | ---------: | -----------:
       1 |          1 |          1 |       321500
       2 |          1 |          1 |       321500
       3 |          2 |       null |         null
       4 |          2 |       null |         null
       5 |          2 |       null |         null
       6 |          3 |       null |         null 


db&lt;>fiddle here