If I have already an inventory management software, do I still need a warehouse management software?

In general, inventory management usually tells warehouse stakeholder the stock quantity or stock value in an entire warehouse or a large location.

Tracking Product up to detail bin location

In a nutshell, Warehouse Management Software (WMS) is a inventory management at micro level. You are often required to register the bin location of a particular product. Most of the time, it is not just the Item or Product code but also other essential information such as production batch number, expiry date, country of origin and etc.

By doing so, you can achieve :-

1. FIFO/FEFO Picking (or any specific picking by criteria)
2. Locating product more efficiently. Picking Slip or using handheld device to direct the order picker with picking route.
3. Cycle counting (daily count by product, without WMS, locating product can be time consuming and less practical to do so)

Compared to using spreadsheet to record such information, a WMS should offer functionality such as producing product movement reports, you can tell exactly which product has been moved by which user with its timestamp. 

Aside from that, most WMS, designed for multi user environment which empower you with security access control which most spreadsheet program cannot offer.

Accuracy

Anyone who run a warehouse would agree that accuracy is high priority if not the top. How to capture information accurately and fast has becoming the key to success. Most WMS work with handheld computer with barcode scanning or RFID reader.

By scanning or reading the ID (whether it is product code, serial number or etc), it is up the business logic and software capability to match with what it suppose to be.

Leveraging on such technology can sometime improve the speed not by certain percentage but number of times.