can anyone drop a link where I could read about differences between custom field with custom list and a custom segment. It seems that using any of them I can extract necessary data in reports so whats the point and benefits using custom segments?

A custom segment can be considered "GL Impacting", so the data will lock automatically by managing the accounting period. They also appear in the "Column" option of financial statements, so you can more easily run comparative reports. I don't think there is a way to get a custom field in the "view columns by" list of a financial statement.

Yes @Joanna Curtis is right custom segments can become GL impacting. You can also convert them into custom records and put custom fields onto the record Then they will be available to segment financial reports and the coveted view by columns for comparative analysis

thanx, more or less is clear now