Dynamodb gsi vs table

I am having trouble understanding what the difference is between a global secondary index and a table. Why would I use a global secondary index, why not just create another table. I have to specify read and write throughput for both. When a write occurs on a table with gsi I have to write to both the table and the index. My question then is why not just create another table instead of a global secondary index? What benefit do I get by using a gsi?