why does parenchyma have large intercellular space?

the intercellular air spaces permit gaseous exchange.

Parenchyma proper, or ground tissue, is usually considered as a tissue lacking in a high degree of functional and structural specialization, and which consists of highly vacuolated thin-walled cells with a minimal amount of cytoplasm. Examples of such tissue are cortical parenchyma of stems and roots as well as the pith of stems. However, parenchyma cells may be highly specialized when they are parts of a complex tissue like xylem parenchyma. Other examples of highly specialized types of parenchyma include assimilation parenchyma of leaves (mesophyll), aerenchyma and storage parenchyma of tubers and seeds.

Parenchyma is often called fundamental, or ground tissue, since it appears as a ground substance in which other tissues, primarily vascular, are embedded. It is also the foundation of the plant inasmuch as the apical meristems and the reproductive cells are parenchymatous in nature. It is also noteworthy to point out that more primitive multicellular non-tracheophytes tend to consist of parenchyma only. Some of the most important activities of the plant, such as photosynthesis, assimilation, respiration, storage, and secretion are primarily based in parenchymatous tissues. While parenchyma cells are simple, primitive, or unspecialized, they also possess the highest degree of developmental plasticity that enables them in specialized circumstances to become transformed into other cell types (e.g. sclerenchyma).

While parenchyma cells are generally thought of as being thin-walled, they sometimes have very thick primary walls as in the case of endosperm cells of date palm, persimmon,Asparagus and Coffea arabica. Mature parenchyma tissues are either close-packed, or may be permeated by a large intercellular space system. Storage parenchyma of fleshy fruits generally has abundant intercellular spaces. In contrast, the endosperm of most seeds contains no (or small) intercellular spaces. The intercellular (or air) spaces reach their highest development in the aquatic angiosperms, both in individual size and in combined volume. In these plants, the air in the spaces serves not only for aeration, but (when they occur in floating leaves or stems) also to give the plants buoyancy and support. The air spaces form an elaborate system that appears to be continuous from the leaf to the root, thereby allowing oxygen to diffuse from its point of highest concentration in the leaves to places of scarcity in the tissues that are lacking chlorophyll.