block can be used to group a block of tasks together. As a sort of arbitrary example, let's say you have a playbook that contains three tasks. In this example, the first two tasks contain the when parameter so that the tasks is only run on managed hosts in the "web" group.

---
- hosts: all
  tasks:
  - name: create the /usr/local/foo directory
    ansible.builtin.file:
      path: /usr/local/foo
      state: directory
    when: inventory_hostname in ( groups['web'] )

  - name: upload foo.txt to /usr/local/foo/foo.txt
    ansible.builtin.copy:
      src: /usr/local/ansible/files/foo.txt
      dest: /usr/local/foo/foo.txt
    when: inventory_hostname in ( groups['web'] )

  - name: update foo.txt owner group mode
    ansible.builtin.file:
      path: /usr/local/foo/foo.txt
      owner: john.doe
      group: admins
      mode: "0770"
...

The block parameter can be used to group the first two tasks together, so that the when parameter only needs to be declared once, which results in a much less repetitive and less error prone playbook.

---
- hosts: all
  tasks:
  - block:
    - name: create the /usr/local/foo directory
      ansible.builtin.file:
        path: /usr/local/foo
        state: directory

    - name: upload foo.txt to /usr/local/foo/foo.txt
      ansible.builtin.copy:
        src: /usr/local/ansible/files/foo.txt
        dest: /usr/local/foo/foo.txt
  
    when: inventory_hostname in ( groups['web'] )

  - name: update foo.txt owner group mode
    ansible.builtin.file:
      path: /usr/local/foo/foo.txt
      owner: john.doe
      group: admins
      mode: "0770"
...

Let's say you have a group of tasks that require become. Without block, you may have two (or more) tasks with the same become settings. In this example, we do not want to use become for the entire playbook since there are tasks to do not require become.

---
- hosts: all
  remote_user: superuser
  tasks:
  - name: create the /usr/local/foo directory
    become: yes
    become_method: sudo
    ansible.builtin.file:
      path: /usr/local/foo
      state: directory
      owner: john.doe
      groups: admins

  - name: upload bar.txt to /usr/local/foo/bar.txt
    become: yes
    become_method: sudo
    ansible.builtin.copy:
      src: bar.txt
      dest: /usr/local/foo/bar.txt
      owner: john.doe
      groups: admins
      mode: '0777'

  - name: append 'Hello World' to bar.txt
    ansible.builtin.lineinfile:
      path: /usr/local/foo/bar.txt
      line: "Hello World"
...

block can be used to use become in a block of tasks which results in a much cleaner, less repetitive and less error prone playbook.

---
- hosts: all
  remote_user: superuser
  tasks:
  - block:
    - name: create the /usr/local/foo directory
      ansible.builtin.file:
        path: /usr/local/foo
        state: directory
        owner: john.doe
        groups: admins

    - name: upload bar.txt to /usr/local/foo/bar.txt
      ansible.builtin.copy:
        src: bar.txt
        dest: /usr/local/foo/bar.txt
        owner: john.doe
        groups: admins
        mode: '0777'

    become: yes
    become_method: sudo

  - name: append 'Hello World' to bar.txt
    ansible.builtin.lineinfile:
      path: /usr/local/foo/bar.txt
      line: "Hello World"
...