This mechanism is part of Japan's tax system. If you are not a Japanese tax resident, different offsetting rules will apply under the tax law of your own country.
How it works
For example, if you have a gain in an account at Brokerage A and a loss in an account at Brokerage B, combining the two can reduce the amount of income subject to tax for that year. Even between two "with withholding" specific accounts, a tax return (see related page) is required if the accounts are held at different brokerages.
Relationship to tax loss carryforward
Any loss that cannot be fully offset within a given year may, in some cases, be carried forward to future years using tax loss carryforward (see related page). Please confirm the eligibility requirements and procedure with a qualified tax accountant.