As a practical matter, the audio announcement by Boko Haram leader Abubakar Shekau that the terrorist organization pledged its allegiance to the Islamic State caliphate doesn't mean very much. A vast distance separates the two terror groups and neither one can send significant aid if one of them gets in trouble.

But as a propaganda coup, both groups score big.

BBC:

In the audio message posted on Saturday, the Boko Haram leader purportedly said: "We announce our allegiance to the caliph... and will hear and obey in times of difficulty and prosperity.

"We call on Muslims everywhere to pledge allegiance to the caliph."

Boko Haram's insurgency has threatened Nigeria's territorial integrity and triggered a humanitarian crisis.

It has carried out frequent bombings that have left thousands dead in Nigeria's north-east and has also attacked targets in the capital, Abuja.

On Saturday, at least five blasts including several suicide bombings in the north-eastern Nigerian city of Maiduguri left at least 50 people dead

[...]

IS has forged links with other militant groups across North Africa and the Arabian Peninsula.

In November Abu-Bakr al-Baghdadi accepted pledges of allegiance from jihadists in Egypt, Libya, Algeria, Yemen and Saudi Arabia.

In January, militants in Afghanistan and Pakistan announced that they were forming an IS "province".

This analysis by the BBC's Jim Muir makes sense:

The announcement by the Boko Haram leader came in a well-produced audio message posted on the internet, with rolling translation from his Arabic to both French and English. Although it was not an elaborate video production like those put out by IS itself, it had many of the IS hallmarks, including an Islamic anthem at the beginning.

The step came as no surprise, given evidence in Boko Haram's propaganda output of growing IS influence on the Nigerian movement, whose ideology and harsh practices mirror those of IS itself.

What it will add up to in practical terms, given Boko Haram's local roots and the geographical distance between them, is hard to see. But from a propaganda point of view, it's a boost for IS, whose presence on the ground in Iraq and Syria is generally stalled and on the defensive.

Boko Haram has been set back on its heels by the coalition of Niger, Kenya, Chad, and Nigerian troops carrying out an offensive against terrorist held towns. But they still appear to be capable of striking against their enemies with suicide attacks and small raids. Boko Haram's numbers are nowhere near those of Islamic State, and the terrorists are incapable of launching the kind of large scale attacks on urban centers that IS is able to do.

But they can still massacre hundreds at a time and disrupt  security in northern Nigeria. The war against Boko Haram will take years with no guarantee of a successful outcome. This union with IS doesn't help matters at all.