Why OOB management preferred? can you do it using blade?
In-band management goes away if the network goes down. When using in-band management, it is possible to render the device unable to be reached from the network, and that requires an on-site visit to correct the situation.
Most enterprise-grade network devices have a serial console port which can be connected to a modem (or some other communication device) which can be used to perform device management over a phone line (or some other distance connection) for out-of-band management.
Not a very precise question, but here goes...
OOBM is generally used in situations where you have a risk of having your production access cut off, and you want to make sure you still have admin access to your environment in that type of situation, to fix whatever went wrong.
Examples include : - management access to a WAN router through a POTS line or 3G modem so you can check on the status of the router if the WAN circuit is down. - management access to a datacenter network even if some major incident has occured. One common example is when you add a vlan to a trunk and forget the "add" keyword, thus cutting off all other vlans including the management one... which is no longer an issue with OOBM. - ILO cards on servers also allow you to access the server even if its production OS is corrupted. The idea is the same.
Blade server chassis typically offer something similar : you have a means to access the Blade Server chassis itself, and from there an interface provides you with functionality similar to that of an ILO card.
In addition to @RonMaupin and @JeremyGibbons' answers, many organizations use a physically separate network to manage their devices to improve device security. The idea is they can connect to the devices using a network that has highly restricted access, and no access from outside the organization (e.g. the Internet). Only administrators are connected to the management network.
Many devices have separate ethernet interfaces just for this purpose. You will see it labeled as "management interface" or "service port." You can configure the device so that it only allows the management interface to gain access to the device configuration (CLI, GUI, etc).