These ARPs are used for Duplicate Address Detection (DAD).
This is common when a DHCP client receives an offer from a DHCP server.
From the DHCP RFC: RFC 2131 Section 4.4.1
The client SHOULD perform a check on the suggested address to ensure
that the address is not already in use. For example, if the client is
on a network that supports ARP, the client may issue an ARP request for
the suggested request. When broadcasting an ARP request for the
suggested address, the client must fill in its own hardware address as
the sender's hardware address, and 0 as the sender's IP address, to
avoid confusing ARP caches in other hosts on the same subnet.
Some operating systems also send these gratuitous ARPs to detect duplicate addresses when statically configured. The operation is OS dependent. This article is a few years old but discusses the behaviour on several versions of Windows.
In address conflict detection, we use what is known as a Gratuitous ARP.
When a system is configured with an IP address either manually or by
DHCP it will send a Gratuitous ARP to insure that another node on the
network is not already configured with this IP address.
The article continued to describe the operation in older versions of the operating system:
In Windows XP and Windows Server 2003 the Gratuitous ARP request is sent
with the Senders MAC filled in with the MAC of the sending system and
the Target MAC set to 0’s, but the Senders and Target IP address are
both set to the address of the sending system. If a conflict is detected
then the defending system replies with its IP and MAC address.
The article then states that the behaviour was changed in later versions of Windows.
Additionally, when a gratuitous ARP is sent by a Windows Vista or
Windows Server 2008, the following change has been made – the SPA field
in the initial request is set to 0.0.0.0. This way the ARP or neighbor
caches of systems receiving this request are not updated. So, if there
is a duplicate IP address, the receivers do not need to have their cache